ENH: add exclude-files input to skip paths from style checking

[mmcky]

Bulk mode currently reviews every lecture under lectures-path with no way to hold files back. Some files should not be style-checked — generated or vendored content, in-progress drafts, _admonition/ style includes and other partials, or lectures deliberately exempted from a rule family — and today the only options are to check them anyway or to not run the action.

This was part of the original request in QuantEcon/meta#226, which asked for exclusion with regex support. It was not carried over when the checker shipped, and it has no home in style-guide either — that repo's schema has per-rule exclusions, which is a different granularity: it answers "which rules never apply", not "which files are never read".

Proposal: a new optional exclude-files input taking a comma-separated list of glob patterns, matched against paths relative to lectures-path, applied in both single and bulk mode so an excluded file is skipped even when named explicitly.

- uses: QuantEcon/action-style-guide@main
  with:
    mode: bulk
    lectures-path: lectures/
    exclude-files: '_static/**,**/draft_*.md,_admonition/**'

Globs are suggested over the regex in #226 because they match how lectures-path is already consumed and are easier to get right in YAML — but regex is a reasonable alternative if the more expressive form is wanted.

Touch points: action.yml (new input), style_checker/action.py (thread through review_single_lecture and review_bulk_lectures), and github_handler.find_lecture_file.