Add basic .xrdb/.Xresources import plugin

Adds basic support for importing .Xresources and .xrdb files as Theme
resources.

Examples of terminal color schemes in .xrdb format can be found here:
https://github.com/mbadolato/iTerm2-Color-Schemes/tree/master/xrdb

Examples of terminal themes in .Xresources format can be found here:
https://github.com/mbadolato/iTerm2-Color-Schemes/tree/master/Xresources

https://terminal.sexy also supports exporting themes is Xresources
format.

.github/workflows/main.yml

@@ -219,10 +219,14 @@ jobs:
         with:
           run: godot_executable/${{ env.GODOT }} --no-window -s plug.gd install
 
+      # Use export to force re-import of files, even though it will fail if we
+      # don't have any export templates installed it should still import the
+      # files first.
       - name: Import files
         uses: GabrielBB/xvfb-action@v1
         with:
-          run: ./godot_executable/${{ env.GODOT }} --editor --quit
+          run: ./godot_executable/${{ env.GODOT }} --export HTML5
+        continue-on-error: true
 
       - name: Run tests
         uses: GabrielBB/xvfb-action@v1