Add project scaffolding

Makefile, Dockerfile, CI workflow, prettier config, manifests for
Chrome (MV3) and Firefox (MV2), source directory structure, and
minimal test suite. All checks pass.

.gitignore

@@ -0,0 +1,27 @@
+# OS
+.DS_Store
+Thumbs.db
+
+# Editors
+*.swp
+*.swo
+*~
+*.bak
+.idea/
+.vscode/
+*.sublime-*
+
+# Node
+node_modules/
+
+# Environment / secrets
+.env
+.env.*
+*.pem
+*.key
+
+# Build output
+dist/
+
+# Yarn
+.yarn-integrity