Move in-tree extensions to top-level folder (#993)

Signed-off-by: Jari Kolehmainen <jari.kolehmainen@gmail.com>

extensions/example-extension/tsconfig.json

@@ -0,0 +1,13 @@
+{
+  "extends": "../../tsconfig.json",
+  "compilerOptions": {
+    "outDir": ".",
+    "module": "CommonJS",
+    "sourceMap": false,
+    "declaration": false
+  },
+  "include": [
+    "../../types",
+    "./example-extension.tsx"
+  ]
+}