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:

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

Start with the deprecation checklist in CONTRIBUTING.md and the existing wasm2js and -sWASM=2 usage. Work through the listed global search, emcc warning, communication, release wait, and removal steps; done means the features are removed after the required deprecation period, unless the deprecation is delayed or aborted.

Written by the indexing model from the issue text.