Mark Sphinx extensions as parallel-safe

This makes it possible to build the whole documentation in parallel
by specifying the `-j <threads>` argument on the `sphinx-build`
command line.

extensions/gdscript.py

@@ -169,3 +169,8 @@ class GDScriptLexer(RegexLexer):
 
 def setup(sphinx):
     sphinx.add_lexer('gdscript', GDScriptLexer())
+
+    return {
+        'parallel_read_safe': True,
+        'parallel_write_safe': True,
+    }

extensions/sphinx_tabs/tabs.py

@@ -339,3 +339,8 @@ def setup(app):
                 app.add_javascript(path)
     app.connect('html-page-context', update_context)
     app.connect('build-finished', copy_assets)
+
+    return {
+        'parallel_read_safe': True,
+        'parallel_write_safe': True,
+    }