Add smart cursor positioning for LaTeX snippet insertion

Implement intelligent cursor positioning inspired by mathURL that
enhances the LaTeX editing experience:

- Automatically selects placeholder text in snippets like \hat{x},
  \frac{a}{b}, allowing users to immediately type replacements
- Positions cursor inside empty braces {} for quick editing
- Jumps to first & in matrix/alignment templates
- Falls back to $$ delimiters or end of text as needed

This significantly improves the UX when using the cheatsheet quick
insert buttons, reducing manual cursor repositioning.

Author: claude