Bazel: Migrate to bzlmod (#1530)

This migrates the bazel integration to the new Bazel dependency system
"bzlmod". bzlmod is becoming mandatory this year (see second sentence
here: https://bazel.build/external/migration).

This is a backwards incompatible migration, directly removing the old
WORKSPACE based approach. Users will have to change how they depend on
bzlmod, however I assume pretty much every user will be happy about it,
because they are forced to use bzlmod anyway or add extra flags to
continue to build with newer Bazel versions. Given that users normally
depend on specific git commits in the old system, they won't be hit with
this until they decide to upgrade emsdk.

The basic principle here is simple: I took everything that WORKSPACE did
and searched an alternative in bzlmod. Some interesting bits:

- We have less worries about multiple versions and people depending on
emscripten multiple times in different ways. This is resolved by the new
system: Bazel first iterates through the MODULE.bazel files recursively,
then finds the minimum version needed for each module and then executes
the module extensions that define repos exactly once at that version. So
no more ifs needed to detect multiple inclusions.
- A bunch of nodejs stuff moves to MODULE.bazel, because that is how the
nodejs module works now. As their module extension gets executed only
once you need to declare everything that you could need before that in
the MODULE.bazel file. A side effect of that is that we have to make a
fake repository when emscripten doesn't have an arm64 binary for linux,
because we can't actually figure that out in MODULE.bazel, so we have to
declare that it always exists and then create one in all cases.

There is a bunch of autoformatter changes in here as well, I could try
to revert them if you prefer.

Closes #1509

bazel/README.md

@@ -2,35 +2,29 @@
 
 ## Setup Instructions
 
-In `WORKSPACE` file, put:
+Support for depending on emsdk with a WORKSPACE file was removed and last available in [emsdk version 4.0.6](https://github.com/emscripten-core/emsdk/tree/24fc909c0da13ef641d5ae75e89b5a97f25e37aa). Now we only support inclusion as a bzlmod module.
+
+In your `MODULE.bazel` file, put:
 ```starlark
-load("@bazel_tools//tools/build_defs/repo:git.bzl", "git_repository")
-git_repository(
-    name = "emsdk",
+emsdk_version = "4.0.6"
+bazel_dep(name = "emsdk", version = emsdk_version)
+git_override(
+    module_name = "emsdk",
     remote = "https://github.com/emscripten-core/emsdk.git",
-    tag = "3.1.64",
     strip_prefix = "bazel",
+    tag = emsdk_version,
 )
-
-load("@emsdk//:deps.bzl", emsdk_deps = "deps")
-emsdk_deps()
-
-load("@emsdk//:emscripten_deps.bzl", emsdk_emscripten_deps = "emscripten_deps")
-emsdk_emscripten_deps(emscripten_version = "3.1.64")
-
-load("@emsdk//:toolchains.bzl", "register_emscripten_toolchains")
-register_emscripten_toolchains()
-```
-The `tag` and `emscripten_version` parameters correspond to the git revision of
-[emsdk 3.1.64](https://github.com/emscripten-core/emsdk/releases/tag/3.1.64). To get access to
-newer versions, you'll need to update those. To make use of older versions, change the
-parameter of `git_repository` and `emsdk_emscripten_deps()`. Supported versions are listed in `revisions.bzl`
-
-Bazel 7+ additionally requires `platforms` dependencies in the `MODULE.bazel` file.
-```starlark
-bazel_dep(name = "platforms", version = "0.0.9")
 ```
 
+You can use a different version of this SDK by changing it in your `MODULE.bazel` file. The Emscripten version is by default the same as the SDK version, but you can use a different one as well by adding to your `MODULE.bazel`:
+
+```
+emscripten_deps = use_extension(
+    "@emsdk//:emscripten_deps.bzl",
+    "emscripten_deps",
+)
+emscripten_deps.config(version = "4.0.1")
+```
 
 ## Building
 
@@ -68,28 +62,29 @@ is the preferred way, since it also unpacks the resulting tarball.
 
 The Emscripten cache shipped by default does not include LTO, 64-bit or PIC
 builds of the system libraries and ports. If you wish to use these features you
-will need to declare the cache when you register the toolchain as follows. Note
+will need to declare the cache in your `MODULE.bazel` as follows. Note
 that the configuration consists of the same flags that can be passed to
-embuilder. If `targets` is not provided, all system libraries and ports will be
+embuilder. If `targets` is not set, all system libraries and ports will be
 built, i.e., the `ALL` option to embuilder.
 
 ```starlark
-load("@emsdk//:toolchains.bzl", "register_emscripten_toolchains")
-register_emscripten_toolchains(cache = {
-    "configuration": ["--lto"],
-    "targets": [
-        "crtbegin",
-        "libprintf_long_double-debug",
-        "libstubs-debug",
-        "libnoexit",
-        "libc-debug",
-        "libdlmalloc",
-        "libcompiler_rt",
-        "libc++-noexcept",
-        "libc++abi-debug-noexcept",
-        "libsockets"
-    ]
-})
+emscripten_cache = use_extension(
+    "@emsdk//:emscripten_cache.bzl",
+    "emscripten_cache",
+)
+emscripten_cache.configuration(flags = ["--lto"])
+emscripten_cache.targets(targets = [
+    "crtbegin",
+    "libprintf_long_double-debug",
+    "libstubs-debug",
+    "libnoexit",
+    "libc-debug",
+    "libdlmalloc",
+    "libcompiler_rt",
+    "libc++-noexcept",
+    "libc++abi-debug-noexcept",
+    "libsockets"
+])
 ```
 
 See `test_external/` for an example using [embind](https://emscripten.org/docs/porting/connecting_cpp_and_javascript/embind.html).