Fix 19623 - HERE string identifier cannot start with Unicode letter

[dkorpel]

p-- assumes the last decoded char was a single code unit, so in case of a Unicode letter, the identifier is scanned starting in the middle of a code point.

[dlang-bot]

Thanks for your pull request and interest in making D better, @dkorpel! We are looking forward to reviewing it, and you should be hearing from a maintainer soon.
Please verify that your PR follows this checklist:

• My PR is fully covered with tests (you can see the coverage diff by visiting the details link of the codecov check)
• My PR is as minimal as possible (smaller, focused PRs are easier to review than big ones)
• I have provided a detailed rationale explaining my changes
• New or modified functions have Ddoc comments (with Params: and Returns:)

Please see CONTRIBUTING.md for more information.

---

If you have addressed all reviews or aren't sure how to proceed, don't hesitate to ping us with a simple comment.

Bugzilla references

Auto-close	Bugzilla	Severity	Description
✓	19623	trivial	HERE string identifier cannot start with Unicode letter

Testing this PR locally

If you don't have a local development environment setup, you can use Digger to test this PR:

dub run digger -- build "master + dmd#14625"

[ibuclaw]

A good example of a fix for stable.

1. Is localised
2. Fixes a bug report.

[dkorpel]

On the other hand, it's old and obscure, so there's no pressure to put it in a release quickly.