metadata: Add vcs-browser URL

Add vcs-browser URL to point to the source code repository.

Author: yakushabb

data/org.stellarium.Stellarium.appdata.xml

@@ -498,6 +498,7 @@
   <url type="translate">https://app.transifex.com/stellarium/stellarium/</url>
   <url type="bugtracker">https://github.com/Stellarium/stellarium/issues</url>
   <url type="help">https://www.stellarium.org</url>
+  <url type="vcs-browser">https://github.com/Stellarium/stellarium</url>
   <content_rating type="oars-1.1">
     <content_attribute id="social-info">moderate</content_attribute>
   </content_rating>