devops: scope the -diff gitattribute to the minified highlight.js

[prql-bot]

.gitattributes marks highlight.js -diff to keep mdbook's 116KB single-line minified bundle out of git grep and git diff output. Git matches a pattern with no / against the basename at any depth, so it also matches grammars/prql-lezer/src/highlight.js — 45 lines of hand-written source added in #6307. Every change to that file renders as Binary files … differ, locally and in GitHub's PR view, so its diff is invisible to review.

Spelling out web/book/theme/highlight.js keeps the minified bundle suppressed and restores the source file's diff. The neighbouring package-lock.json entry is unanchored for the opposite reason — the repo has several of them and they should all be suppressed — so it stays as it is.

Verification

git check-attr on this branch, with web/book/theme/highlight.js still carrying diff: unset and the lezer source no longer doing so:

$ git check-attr -a web/book/theme/highlight.js grammars/prql-lezer/src/highlight.js
web/book/theme/highlight.js: diff: unset
web/book/theme/highlight.js: text: auto
web/book/theme/highlight.js: eol: lf
grammars/prql-lezer/src/highlight.js: text: auto
grammars/prql-lezer/src/highlight.js: eol: lf

Replaying #6307's own commit shows what review missed at the time:

$ # on main
$ git diff 800d919d^..800d919d -- grammars/prql-lezer/src/highlight.js
diff --git a/grammars/prql-lezer/src/highlight.js b/grammars/prql-lezer/src/highlight.js
index 068b8a78..06505ff2 100644
Binary files a/grammars/prql-lezer/src/highlight.js and b/grammars/prql-lezer/src/highlight.js differ

$ # on this branch
$ git diff 800d919d^..800d919d -- grammars/prql-lezer/src/highlight.js
diff --git a/grammars/prql-lezer/src/highlight.js b/grammars/prql-lezer/src/highlight.js
index 068b8a78..06505ff2 100644
--- a/grammars/prql-lezer/src/highlight.js
+++ b/grammars/prql-lezer/src/highlight.js
@@ -1,7 +1,14 @@
 import { styleTags, tags as t } from "@lezer/highlight";
...

No regression test: .gitattributes is git configuration with no code path to exercise, and git check-attr is the only assertion available.