chore: use REUSE instead of non-standard license declaration

reference/bitmap assets/REUSE.toml

@@ -0,0 +1,8 @@
+version = 1
+
+[[annotations]]
+path = "Nycticorax_nycticorax12.jpg"
+precedence = "override"
+SPDX-FileComment = "Source: https://commons.wikimedia.org/wiki/File:Nycticorax_nycticorax12.jpg"
+SPDX-FileCopyrightText = "Calibas"
+SPDX-License-Identifier = "CC-BY-SA-3.0"