fix: missing flag from regex literal in corner case

[psve]

In the specific corner case where a regex contains a flag but is not followed by any symbols, the ast.RegExpLiteral.Literal value would drop (part of) that flag.

This happens because the p.next() call during the parsing of the flag wouldn't actually advance the p.chrOffset, and thus endOffset would be erroneously adjusted.

The fix is just to use the current p.chrOffset and then call p.next() afterwards.

[psve]

@stevenh , I removed the erroneous import, the linter checks should pass now.

[stevenh]

Thanks for the PR @psve 🥇