Change the extension's repo URL to the Github mirror

When packaging the extension, vsce will use this URL to rewrite links to
images and markdown files in README.md.

Change-Id: I88fa9f4b2c77f812f8f8682392b0b518f2b8729b
Reviewed-by: Marcus Tillmanns <marcus.tillmanns@qt.io>

package.json

@@ -8,7 +8,7 @@
   "publisher": "theqtcompany",
   "repository": {
     "type": "git",
-    "url": "https://codereview.qt-project.org/qt-labs/vscodeext"
+    "url": "https://github.com/qt-labs/vscodeext"
   },
   "bugs": {
     "url": "https://bugreports.qt.io/projects/VSCODEEXT"