10$: JS string decoding routine

[strager]

Write a routine which decodes escape sequences in string literal tokens.

Caveats:

• String span might not start or end with ' or " (due to error recovery)
• Escape sequences might be invalid

No diagnostics should be reported by the routine. (Diagnostics should have been reported already when lexing the string literal.)

Memory should be allocated in a user-provided Memory_Resource, probably using Vector.

Decoded string should be (possibly-invalid) UTF-8.