Fix bundled editor CSS package export

[dawn (dawNotPoi)]

Summary

• export the existing bundled editor stylesheet through the package exports map
• add an esbuild smoke regression that resolves the CSS from the generated package

Tests

• npm run build-lsp
• npm run build-monaco-editor
• npm test
• npm run compile --prefix webpack-plugin
• npm run package-for-smoketest
• npx prettier --check package.json test/smoke/package-esbuild.ts test/smoke/esbuild/editor-main-css.js

Fixes #5404

[dawn (dawNotPoi)]

@microsoft-github-policy-service agree