chore: add reuse licence checking (#6659)

This PR aims to add very basic REUSE license checking
https://reuse.software.

My intention is not to sort out all errors in this PR, but instead sort
that out over multiple incrementals PR going forward in an undisruptive
way as possible.

.github/workflows/reuse.yml

@@ -0,0 +1,21 @@
+# SPDX-FileCopyrightText: 2025 Rasmus Karlsson <rasmus.karlsson@pajlada.com>
+# SPDX-License-Identifier: CC0-1.0
+---
+name: REUSE license check
+
+on:
+  pull_request:
+
+permissions:
+  contents: read
+
+jobs:
+  reuse:
+    runs-on: ubuntu-latest
+    steps:
+      - uses: actions/checkout@v4
+
+      - name: REUSE license check
+        uses: fsfe/reuse-action@v6
+        # I don't want to scare contributors yet, but I want to see progress in actions.
+        continue-on-error: true