Add open-external-editor *module* to .gitignore

Because the setting to enable it is in the project.godot, there's no way
to keep that out of source control, but there is a way to keep this out
so here it is.

.gitignore

@@ -8,6 +8,7 @@ nohup.out
 recordings
 ai
 release
+addons/open-external-editor
 
 ._*
 ehthumbs.db