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

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

extensions/example-extension/package.json

@@ -0,0 +1,12 @@
+{
+  "name": "extension-example",
+  "version": "1.0.0",
+  "description": "Example extension",
+  "main": "example-extension.js",
+  "lens": {
+    "metadata": {},
+    "styles": []
+  },
+  "dependencies": {
+  }
+}