feat: add VS Code web extension package

[hongjr03]

Summary

• add browser-host VS Code extension entry backed by WASM worker runtime
• add @vscode/test-web smoke test for web host activation
• add package:web to CI and release workflows with playground WASM caching
• hide unsupported browser-runtime capabilities for formatter and Qihe execution

Verification

• npm test
• npm run test:web
• npm run package:web
• npm run build --prefix playground
• npm run build:embed --prefix playground
• git diff --check --cached