Add comprehensive test suite for completion, highlight, and indent

- Add completion-tests.lisp: word char classification, path detection,
  prefix matching, common prefix finding (24 tests)
- Add highlight-tests.lisp: whitespace/constituent detection, Lisp
  tokenizer, paren matching, display width (30 tests)
- Add indent-tests.lisp: whitespace skipping, indent calculation,
  reindentation (19 tests)
- Add 'check' as alias for 'test' target in Makefile
- Total test suite now has 212 checks (up from 81)

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Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>