Exclude asset-sources from git archive output

This will roughly halve the size of the download when people fetch this from
the Godot Asset Library.

Author: wjt

.gitattributes

@@ -4,3 +4,6 @@
 # Exclude GitHub Actions-related files from published asset
 /.github                  export-ignore
 /asset-template.json.hb   export-ignore
+
+# Exclude artwork sources
+/asset-sources            export-ignore