fix: complete monaco dependency build migration

[exKAZUu]

Follow-up to #6.

Summary

• Switch the package build from tsup to build-ts lib and remove the obsolete tsup config.
• Add the public entrypoint use client directive so the minified build-ts output preserves the client-component marker that the old tsup banner injected.
• Update installation docs to reflect that monaco-editor is now installed with this package instead of required as a peer install.
• Strengthen the Next.js e2e fixture so it requires built package artifacts, checks package export files, resolves app imports through the built dist entry, and type-checks against generated declarations when dist exists.
• Keep the package build scoped to yarn test:e2e:next, where the Next fixture needs the generated dist package surface.
• Remove the direct typescript dev dependency and rely on @typescript/native-preview / tsgo for project type checking.
• Resolve lint warnings and make clean-checkout linting pass before generated dist artifacts exist.
• Use the repo-local build/core script for yarn build so CI does not depend on a global wb binary.

Why

• PR fix: own monaco editor dependency #6 moved monaco-editor into runtime dependencies, but the merged branch still used tsup even though the PR body described the build-ts migration.
• Keeping the client directive in source makes the emitted ESM and CJS entrypoints compatible with React client hooks after replacing the bundler.
• The e2e app should validate the same package surface consumers receive: package exports, generated declarations, and built JS files.
• CI runs from a clean checkout, so lint and build scripts must not rely on locally generated dist files or globally installed WillBooster tooling.

Testing

• yarn typecheck
• yarn build
• yarn lint
• yarn check-for-ai
• yarn test:e2e:next
• git diff --check
• Verified clean-checkout lint behavior by temporarily moving dist outside the repository and running yarn lint.
• Verified minified dist/index.js and dist/index.cjs still start with "use client".
• Verified generated dist/index.js, dist/index.cjs, and dist/index.d.ts exist.
• Verified generated output has no dist/src or dist/node_modules.
• node -e "const m=require('./dist/index.cjs'); console.log(typeof m.default, typeof m.Editor, typeof m.DiffEditor, typeof m.loader, typeof m.useMonaco)"
• node --input-type=module -e "const m=await import('./dist/index.js'); console.log(typeof m.default, typeof m.Editor, typeof m.DiffEditor, typeof m.loader, typeof m.useMonaco)"
• Pre-push hook: check passed with 0 warnings and 0 errors.
• GitHub Actions CI passed on the latest branch state.

Notes

• next build e2e/next-app prints a warning that some Next.js TypeScript features require the standard typescript package because it detects @typescript/native-preview; the build and Playwright e2e still pass.

[gemini-code-assist]

Code Review

This pull request transitions monaco-editor from a peer dependency to a direct dependency and replaces the tsup build system with build-ts. It also adds the 'use client'; directive to the main entry point for React Server Components compatibility. Feedback was provided regarding the phrasing of dependency ownership in the README and a potential versioning error concerning Next.js.