License CC:T according to the REUSE specification (#1351)

This adds SPDX license headers to all source code files, following the
REUSE[1] specification. This does not include any asset files (such as
generated JSON files, or textures). While REUSE does support doing so
with ".license" files, for now we define these licences using the
.reuse/dep5 file.

[1]: https://reuse.software/

buildSrc/build.gradle.kts

@@ -1,3 +1,7 @@
+// SPDX-FileCopyrightText: 2022 The CC: Tweaked Developers
+//
+// SPDX-License-Identifier: MPL-2.0
+
 plugins {
     `java-gradle-plugin`
     `kotlin-dsl`