Add file handling (#1736)

* Add file handling
https://github.com/WICG/file-handling/blob/master/explainer.md#example

* Only trigger on `.excalidraw` for now

public/manifest.json

@@ -17,5 +17,13 @@
   "start_url": ".",
   "display": "standalone",
   "theme_color": "#000000",
-  "background_color": "#ffffff"
+  "background_color": "#ffffff",
+  "file_handlers": [
+    {
+      "action": "/",
+      "accept": {
+        "application/vnd.excalidraw+json": [".excalidraw"]
+      }
+    }
+  ]
 }