Add drop feedback UX (#179434)

For #179430

Adds two new UX components:

- An inline progress icon shown when a drop operation takes over 500ms. This replaces the notification. You can click on it to cancel the drop

- Post drop, a drop feedback icon that lets you drop the file in a different way. This lets you drop the file as plain text for instance instead of as a markdown link

extensions/markdown-language-features/package.json

@@ -16,7 +16,8 @@
     "Programming Languages"
   ],
   "enabledApiProposals": [
-    "documentPaste"
+    "documentPaste",
+    "dropMetadata"
   ],
   "activationEvents": [
     "onLanguage:markdown",