[PSA] Plan to remove WASM_BIGINT=0

[Sam Clegg]

Wasm/bigint integration is now available in even the oldest engines that emscripten can target so disabling doesn't make much sense these days.

See https://github.com/emscripten-core/emscripten/issues/27565.

If you are a user of these flags please reply here or comment on the issue.

Note that IIUC the main user of WASM_BIGINT=0 is wasm2js (-sWASM=0), which we also hope to deprecate at some point in the future, but will consider separately. 

cheers,
sam