proposal-regexp-modifiers: Tests for syntax errors

[guijemont]

This covers the syntax error cases from #3756.
This is an updated version of #3758 addressing review comments.

[guijemont]

The main issue why this is still a draft, is that the templated test generator has some issues.
In particular, we want to use it with templates such as:

/(?/*{ subpattern-add-modifiers }*/-/*{ subpattern-remove-modifiers }*/:a)//*{ global-modifiers }*/;

But the generator cannot replace /*{ global-modifiers }*/ as it seems to consider that //*{ global-modifiers }*/; is a comment. In otherwords, the generator needs to learn about how to handle comments that immediately follow a regex literal.

[guijemont]

The main issue why this is still a draft, is that the templated test generator has some issues. In particular, we want to use it with templates such as:

/(?/*{ subpattern-add-modifiers }*/-/*{ subpattern-remove-modifiers }*/:a)//*{ global-modifiers }*/;

But the generator cannot replace /*{ global-modifiers }*/ as it seems to consider that //*{ global-modifiers }*/; is a comment. In otherwords, the generator needs to learn about how to handle comments that immediately follow a regex literal.

I created issue #3808 to report the problem.

[rbuckton]

This looks good as far as invalid syntax coverage is concerned, at least once #3808 is resolved.

[ptomato]

Closing in favour of #4016.