Fix invalid regex (fixes #159)

[slevithan]

See extended details at #159, but essentially, \xHH for values above 7F doesn't work the same in Oniguruma (the regex engine used for TextMate grammars) as in other flavors of regex. As a result, a standalone unenclosed \xff like the one corrected here is an invalid UTF-8 encoded byte value, rather than a valid code point value as it would be if using the enclosed version \x{ff}.

This error is currently leading to highlighting bugs with some input strings (code points above FF are not matched by this negated range), and preventing the Hack grammar from working at all with Shiki's JS engine.

The adjacent \x7f that I also changed was already valid, but I changed it for consistency.

Note that this was the only place the invalid \\x7f-\\xff appeared, but the valid/correct version \\x{7f}-\\x{ff} already appears 28 times in the same file.

[slevithan]

@PranayAgarwal, is anything else needed for this to land?

[PranayAgarwal]

Hey @slevithan, this looks good. Unfortunately I don't work at Slack/Salesforce anymore and don't have maintainer access to this repo, so my approval doesn't count. 🫤

[PranayAgarwal]

Let me message some people to try and find you another reviewer.