chore: vendor Mitt & update project structure (#6209)

* chore: vendor Mitt into src/common/third-party

As discussed in #6203 we need to vendor our common dependencies in so
that when we ship an ESM build all imports point to file paths and do
not rely on Node resolution (e.g. a browser does not understand `import
mitt from 'mitt'`).

tsconfig.base.json

@@ -0,0 +1,12 @@
+{
+  "compilerOptions": {
+    "esModuleInterop": true,
+    "allowJs": true,
+    "checkJs": true,
+    "target": "ESNext",
+    "moduleResolution": "node",
+    "declaration": true,
+    "declarationMap": true,
+    "resolveJsonModule": true
+  }
+}