[WebAssembly] Add half-precision feature (#90248)

This currently only defines a constant, but in the future will be used
to gate builtins for experimenting and prototyping half-precision
proposal (https://github.com/WebAssembly/half-precision).

GitOrigin-RevId: d9fd0ddef38bb9d5cce7300ff820272183c09fcd
3 files changed