Invalid unicode escapes in regexp are not a syntax error

[Earlopain]

For example:

/\u{21!3}/

$ ruby -c test.rb
Syntax OK

$ ruby --parser=parse.y -c test.rb
ruby: test.rb:1: invalid Unicode list: /\u{21!3}/ (SyntaxError)

Prism knows this is invalid but intentionally ignores the error:

prism/src/prism.c

Lines 9810 to 9814 in 3340d87

	if (flags & PM_ESCAPE_FLAG_REGEXP) {
	// If this is a regular expression, we are going to
	// let the regular expression engine handle this
	// error instead of us.
	pm_buffer_append_bytes(regular_expression_buffer, start, (size_t) (parser->current.end - start));

This is because this regex is valid (invalid escape after comment in extended mode):

/# \u{21!3}/x

At runtime everything is right but it would be nice to also report this at parse-time.