Can we completely remove WASM_BIGINT=0

As of #27542 emscripten no longer supports targeting engines without Wasm BigInt integration.

However the WASM_BIGINT=0 is being kept around to enable `-sWASM=0` (i.e. wasm2js).

If we were to add bigint support for wasm2js output then we could completely drop support for WASM_BIGINT=0 from emscripten.

The question is, are users of wasm2js deploying to engines without Bigint support?

This issue will act as the deprecation plan for WASM_BIGINT=0.

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