Add GDScript .editorconfig rules

- Uniformize `.gd` unit test files indentation to tabs (where needed)

modules/gdscript/tests/scripts/parser/errors/class_name_after_annotation.gd

@@ -3,4 +3,4 @@
 class_name HelloWorld
 
 func test():
-    pass
+	pass

modules/gdscript/tests/scripts/parser/errors/double_dictionary_comma.gd

@@ -1,2 +1,2 @@
 func test():
-    var dictionary = { hello = "world",, }
+	var dictionary = { hello = "world",, }

modules/gdscript/tests/scripts/parser/errors/match_multiple_variable_binds_in_branch.gd

@@ -1,4 +1,4 @@
 func test():
-    match 1:
-        [[[var a]]], 2:
-            pass
+	match 1:
+		[[[var a]]], 2:
+			pass