Support UTF string encodings

[guybedford]

This supports decoding UTF encodings like "\u{...}" in all JS strings.

To avoid needing to write a custom parser, the JS engine parser is invoked safely using eval, based on the knowledge that the parsed range is definitely a JS string.

This implements string decoding for the string cases of reexport specifier strings, export strings, and require strings.

This does not add support for identifier encodings or filtering for non-surrogate pairs.

[nicolo-ribaudo]

I initially thought about using JSON.parse + custom handling of \u{..}, but eval is definitely better for once!