Migrate koenig-lexical to TypeScript

[kevinansfield]

Summary

• Rename all .js/.jsx files to .ts/.tsx across src/, test/, and demo/
• Add TypeScript configuration (tsconfig.json, eslint.config.js with @typescript-eslint)
• Add type annotations throughout: component props, hooks, utility functions, Lexical node types
• Replace deprecated tseslint.config() with defineConfig() from eslint/config
• Create proper interfaces for untyped dependencies (Emoji for emoji-mart, SimpleMDEInstance/CodeMirrorInstance for kg-simplemde)
• Add test/types.d.ts with Window augmentation for Lexical editor in e2e browser context
• Remove all eslint-disable comments except intentional react-hooks/exhaustive-deps for mount-only effects