refactor(vimsplain): mode-based handlers and testing architecture

[oller]

Summary

• Property-Based Testing: Added fast-check to fuzz the parser with millions of synthetic command combinations to catch edge cases before refactoring.
• Integration Testing: Added tests that run commands against an actual headless CodeMirror instance (via @replit/codemirror-vim) to verify vimsplain's explanations perfectly map to the state changes in CodeMirror.
• Architecture Refactoring: The core parser was completely refactored from a monolithic while loop into a Mode-based Handler architecture, significantly simplifying vimsplain.ts.
• Mode Handlers: Logic extracted to src/handlers/normal.ts, insert.ts, visual.ts, command.ts, and search.ts.

Test Plan

• All 331 tests run completely green
• Test coverage for the vimsplain package is at 97.68% overall
• Linting and typechecks succeed

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[changeset-bot]

🦋 Changeset detected

Latest commit: 14f54cb

The changes in this PR will be included in the next version bump.

This PR includes changesets to release 1 package

Name	Type
vimsplain	Minor

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