Tracking issue for the deprecation and removal of wasm2js (AKA `WASM=0`)

This would also include the removal of -sWASM=2 (which builds both JS and wasm versions of a program in a single command)>

The process for deprecating and removing features from emscripten is laid out in https://github.com/emscripten-core/emscripten/blob/main/CONTRIBUTING.md#deprecating-settings-and-features.

At any point in the process we may decide to delay or abort the deprecation and close this issue.

Deprecation checklist:

• Open an issue (this)
• Add "deprecation" warning to emcc
• Perform global search for usage in public repos
• Send message to emscripten-discuss
• Wait at least 4 releases or 2 months
• Remove feature from codebase