[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

[tlively]

Nice!

[tlively]

Alternatively:

Suggested change

	PassRunner::Ordering ordering;
	ordering.first = (i == firstDefault);
	ordering.last = (i == lastDefault);
	PassRunner::Ordering ordering{i == firstDefault, i == lastDefault};

[tlively]

If you enable GC here, the printed output should be the same as with -all, so you could use one fewer check prefix.

[kripken]

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

binaryen/src/ir/subtype-exprs.h

Lines 415 to 423 in f8531ef

	void visitContNew(ContNew* curr) { WASM_UNREACHABLE("not implemented"); }
	void visitContBind(ContBind* curr) { WASM_UNREACHABLE("not implemented"); }
	void visitSuspend(Suspend* curr) { WASM_UNREACHABLE("not implemented"); }
	void visitResume(Resume* curr) { WASM_UNREACHABLE("not implemented"); }
	void visitResumeThrow(ResumeThrow* curr) {
	WASM_UNREACHABLE("not implemented");
	}
	void visitStackSwitch(StackSwitch* curr) {
	WASM_UNREACHABLE("not implemented");

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.