Add wasm support to IndexOfAnyAsciiSearcher

[stephentoub]

I cherry-picked @MihaZupan's commit and just substituted in the new methods @radekdoulik merged.

[dotnet-issue-labeler]

I couldn't figure out the best area label to add to this PR. If you have write-permissions please help me learn by adding exactly one area label.

[radekdoulik]

LGTM. We can improve it once I will add arithmetic right shift to packed simd class.

[stephentoub]

Thanks, @radekdoulik. Are you ok merging this one and the json one and then seeing where everything lands after that?

[radekdoulik]

Yes, I am looking forward to that. The SIMD is now enabled by default for wasm AOT builds, so we should see changes in the dotnet/perf-autofiling-issues.

[MihaZupan]

Some rough numbers I'm seeing in a stopwatch-style benchmark, IndexOfAnyAsciiByteValues Length=1000 goes from ~16 to ~1.5 microseconds.
On the same machine with regular X86, it takes ~310 ns with AVX2 and ~500 with Vector128 paths.