Add a string lowering mode disallowing non-UTF-8 strings (#6861)

The best way to lower strings is via the "magic imports" API that uses
the names of imported string globals as their values. This approach only
works for valid UTF-8 strings, though. The existing
string-lowering-magic-imports pass falls back to putting non-UTF-8
strings in a JSON custom section, but this requires the runtime to
support that custom section for correctness. To help catch errors early
when runtimes do not support the strings custom section, add a new pass
that uses magic imports and raises an error if there are any invalid
strings.

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