Create .gitattributes

Add syntax highlighting for .lissp on GitHub

This should be using the Common Lisp highlighter, which should be compatible enough to be usable. Lissp syntax is deliberately similar.

Author: gilch

.gitattributes

@@ -0,0 +1,4 @@
+# This configuration file is part of the Hissp source code repository,
+# copyright 2025 Matthew Egan Odendahl
+# SPDX-License-Identifier: Apache-2.0
+*.lissp linguist-language=lisp linguist-vendored