Use emsdk as external bazel dependency (#766)

* Makes provided bazel rules look up @emsdk workspace instead of local workspace
* Uses system-specific emscripten binaries instead of defaulting to linux
* Provides macros for loading emsdk dependencies (nodejs and emscripten binaries)
* Unhardcodes paths in bazel rules and .sh wrappers
* `update_bazel_workspace.sh` now updates `revisions.bzl`
* `emscripten_deps()` can be fed with specific emscripten version
* Adds external usage test

Addresses #650 and #696

bazel/BUILD

@@ -0,0 +1,44 @@
+package(default_visibility = ['//visibility:public'])
+
+config_setting(
+    name = "linux",
+    constraint_values = [
+        "@platforms//os:linux",
+        "@platforms//cpu:x86_64",
+    ],
+)
+
+config_setting(
+    name = "macos",
+    constraint_values = [
+        "@platforms//os:macos",
+        "@platforms//cpu:x86_64",
+    ],
+)
+
+config_setting(
+    name = "windows",
+    constraint_values = [
+        "@platforms//os:windows",
+        "@platforms//cpu:x86_64",
+    ],
+)
+
+alias(
+    name = "binaries",
+    actual = select({
+        ":linux": "@emscripten_bin_linux//:all",
+        ":macos": "@emscripten_bin_mac//:all",
+        ":windows": "@emscripten_bin_win//:all",
+    }),
+)
+
+alias(
+    name = "node_modules",
+    actual = select({
+        ":linux": "@emscripten_npm_linux//:node_modules",
+        ":macos": "@emscripten_npm_mac//:node_modules",
+        ":windows": "@emscripten_npm_win//:node_modules",
+    }),
+)
+