Intent to prototype: WebAssembly exception handling proposal

[Asumu Takikawa]

Summary: We intend to prototype the exception handling proposal for
WebAssembly. The proposal adds a general exception handling facility,
allowing languages that target Wasm to compile to a native exception
feature rather than using JavaScript's exceptions.

Much of the implementation has already landed in the Baseline Wasm
compiler, and implementing it in optimizing tiers is in progress.

Bug: https://bugzilla.mozilla.org/show_bug.cgi?id=1695715 (meta)

Standard: https://github.com/WebAssembly/exception-handling

The specification is in phase 2 (proposal spec text available) of the Wasm
standardization process.

Platform coverage: All.

Preference: For the JS shell, there is an --enable-wasm-exceptions
compilation flag and --wasm-exceptions runtime flag to enable the
feature. No browser preference yet.

DevTools bug: N/A

Other browsers: Staged for origin trial in Chrome v90:
https://www.chromestatus.com/feature/4756734233018368

web-platform-tests: Test suite for Wasm specification is still
pending. This is a requirement for phase 3 (implementation phase).

Cheers,
Asumu