Move Emscripten-related files to Platforms/Emscripten

[hoodmane]

And leave a deprecated Tools/wasm/emscripten/__main__.py behind for backwards-compatibility.

Emscripten version of #143941.

Discussed at https://discuss.python.org/t/moving-to-a-new-platforms-directory/105215/ .

Linked PRs

• gh-145176 Move Emscripten files into Platforms/emscripten #145806
• [3.14] gh-145176 Move Emscripten files into Platforms/emscripten (GH-145806) #146043
• gh-145176: Update CODEOWNERS for Emscripten migration to Platforms directory #149544
• [3.15] gh-145176: Update CODEOWNERS for Emscripten migration to Platforms directory (GH-149544) #149550
• [3.14] gh-145176: Update CODEOWNERS for Emscripten migration to Platforms directory (GH-149544) #149559