[Strings] Add a string-builtins feature, and lift/lower automatically when enabled

[kripken]

This makes string optimizations happen automatically when --enable-string-builtins (or -all).

The lifting/lowering happen globally, at optimal times in the pipeline, so even -O3 -O3 -O3 will only lift once and lower once, avoiding overhead as in #7540 which this replaces.

TODO: document in optimizer cookbook

[kripken]

Hmm, this PR fails many tests because of these TODOs for typed continuations:

https://github.com/WebAssembly/binaryen/blob/f8531ef56e61cb68815c835b294777fbd97f3b0a/src/ir/subtype-exprs.h#L415-L423

This problem becomes noticeable in this PR because we have tests that use -all on continuations code, and now we run StringLowering, which uses SubtypingDiscoverer, so we try to operate on those instructions.

So this PR is blocked on those TODOs.

[tlively]

I can take a look at those TODOs later today, unless you plan on working on them first.

[kripken]

Thanks, I don't think I'd have time today myself.