[Bazel] Add an :all target (#1295)

Having an :all target is the most future-proof way here.

Closes #1294.

bazel/emscripten_deps.bzl

@@ -8,6 +8,11 @@ def _parse_version(v):
 BUILD_FILE_CONTENT_TEMPLATE = """
 package(default_visibility = ['//visibility:public'])
 
+filegroup(
+    name = "all",
+    srcs = glob(["**"]),
+)
+
 filegroup(
     name = "includes",
     srcs = glob([