Add copyright notice to MPL-2 header

Author: Bromeon

.github/other/licenserc.yml

@@ -6,7 +6,14 @@
 
 header:
   license:
-    spdx-id: MPL-2.0
+    spdx-id: 'MPL-2.0'
+    # Note: There would be fields 'copyright-owner', 'copyright-year' and 'software-name', however these infos are not substituted into
+    # the MPL-2.0 header, because the latter needs additional (external) copyright. Thus, content is overridden manually.
+    content: |
+      Copyright (c) godot-rust; Bromeon and contributors.
+      This Source Code Form is subject to the terms of the Mozilla Public
+      License, v. 2.0. If a copy of the MPL was not distributed with this
+      file, You can obtain one at https://mozilla.org/MPL/2.0/.
 
   paths:
     - '**/*.rs'