v2.3.1: feat: add development linter for semantic HTML and accessibility checks

- Introduced a new dev linter that flags common HTML and accessibility issues, including multiple <h1> tags, skipped heading levels, missing alt attributes, and conflicting helper words.
- Added a new JavaScript file `vantaui-lint.js` for linting functionality and `vantaui-lint-rules.js` for mutex rules.
- Updated documentation to include usage instructions for the linter.
- Enhanced the app frame behavior to ensure proper scrolling and sticky positioning of headers and footers.
- Adjusted CSS for app frame and header to improve layout and scrolling behavior.
- Updated spacing tokens to define header height for consistent layout.