Non-UTF string builtins constants?

Discussing with @tlively , we thought it would be good to take the string builtins feature part of #7601 and land that, with the idea that the feature would add validation of string constants: non-utf8 ones would error.

I have code for that in sb.onlyfeat in my fork, however, I see that we can't parse such non-utf8 constants anyhow, e.g.

(module
  (import "\'" "unpaired high surrogate \ED\A0\80 " (global $bad (ref extern)))
)

$bin/wasm-opt test/lit/validation/string-builtins.wast 
Fatal: test/lit/validation/string-builtins.wast:2:52: error: expected import name

So I think that would need to be fixed first? (meanwhile, no one is at risk of trying to use such code, at least...)

Start by reproducing the parser failure with $bin/wasm-opt and the example in test/lit/validation/string-builtins.wast. Read the string builtins work in sb.onlyfeat and determine how non-UTF-8 string constants should be parsed and validated; done means the relevant test input is accepted or rejected with the intended validation behavior.