This CL adds two experimental JS builtins to convert between
i16 Wasm GC and JS strings. This is a non-standard experimental
feature only available with the flag --wasm-gc-js-interop.

WebAssembly.experimentalConvertArrayToString(array, start, count)
Convert the `count`-many WTF-16 code units starting at index `start`
into a JS string. Throws a TypeError if `array` is not an i16 array,
or if `start` and `count` are not numbers or not in range.

WebAssembly.experimentalConvertStringToArray(string, sampleArray)
Convert `string` to an i16 array. The `sampleArray` parameter needs
to be an arbitrary i16 array, which is only used to extract the rtt.
Throws a TypeError if `string` is not a string or `sampleArray` is not
an i16 array.

Change-Id: I7ac2f6bd89b8f638427f61da1bb01ccba90d735b
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/3642301
Commit-Queue: Tobias Tebbi <tebbi@chromium.org>
Reviewed-by: Jakob Kummerow <jkummerow@chromium.org>
Cr-Commit-Position: refs/heads/main@{#80505}