feat(warren): support mbtx browser entries

[Yoorkin]

Summary

• support standalone .mbtx browser entries with warren dev and warren build
• keep the existing package-directory workflow unchanged
• reject server options for browser-only .mbtx entries
• add concise documentation and a single-file counter example
• bump Warren to 0.3.3
• add no C FFI

CI fixes

• apply the formatting required by the current stable MoonBit toolchain
• pin PostCSS to 8.5.26 because 8.5.27 ships a broken TypeScript declaration

Validation

• moon check --warn-list +unnecessary_annotation (0 errors; 6 existing warnings)
• moon test (95 JS tests and 32 native tests passed)
• npm test in vite-plugin
• warren build page.mbtx
• warren dev page.mbtx --direct
• warren dev --browser-entry main against examples/counter
• GitHub Actions: stable-build, vite-plugin, e2e, and typo-check passed