Add GDScript .editorconfig rules

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

modules/gdscript/tests/scripts/analyzer/errors/enum_preload_unnamed_assign_to_named.gd

@@ -1,4 +1,5 @@
 enum MyEnum { VALUE_A, VALUE_B, VALUE_C = 42 }
+
 func test():
 	const P = preload("../features/enum_value_from_parent.gd")
 	var local_var: MyEnum

modules/gdscript/tests/scripts/analyzer/errors/outer_class_lookup.gd

@@ -1,12 +1,12 @@
 class A:
-    class B:
-        func test():
-            print(A.B.D)
+	class B:
+		func test():
+			print(A.B.D)
 
 class C:
-    class D:
-        pass
+	class D:
+		pass
 
 func test():
-    var inst = A.B.new()
-    inst.test()
+	var inst = A.B.new()
+	inst.test()

modules/gdscript/tests/scripts/analyzer/errors/return_null_in_void_func.gd

@@ -1,2 +1,2 @@
 func test() -> void:
-  return null
+	return null

modules/gdscript/tests/scripts/analyzer/errors/return_variant_in_void_func.gd

@@ -1,4 +1,4 @@
 func test() -> void:
-  var a
-  a = 1
-  return a
+	var a
+	a = 1
+	return a