fix(bash) match a function header only at a word boundary

[MaciejZet]

Refs #4529

Changes

The Bash function rule could start in the middle of a word. On a long word it retried the name from every character while looking for () {, so the cost grew with the square of the input. A 64 KB word took about 2.3 s; with the word-boundary anchor the same input finishes in under a millisecond.

foo() {, my_func () {, and a header after another command still highlight the function name. This is one grammar from #4529, so the issue stays open.

Checklist

• Added markup tests, or they don't apply here because...
• I have read and followed our AI-assisted contributions policy (human review, no slop)