implement very basic highlighting

tests/test_highlight

@@ -0,0 +1,22 @@
+#!/bin/sh
+
+# `tree-sitter test` doesn't work to validate syntax highlighting for two reasons:
+# - it identifies a file's language only by its filetype (not shebang/regex), but nix-shell has no filetype
+# - it requires the language to support comments, but nix-shell doesn't have those
+#
+# hence this custom testing tool.
+#
+# usage: test_highlight <TEST_NAME>
+#
+# upon execution, performs this sequence:
+# 1. highlights the file at `highlight/<TEST_NAME>`.
+# 2. asserts the result's body against `hightlight/<TEST_NAME>.spec.html`
+
+here=$(dirname $(realpath $0))
+input_file="$here/highlight/$1"
+spec_file="$here/highlight/$1.spec.html"
+
+# cd into this directory before highlighting because paths specified in the tree-sitter config are relative to PWD
+cd "$here" && XDG_CONFIG_HOME="$here/config" tree-sitter highlight --scope "source.nix_shell" --html "$input_file" | \
+  htmlq 'body' | \
+  diff -u "$spec_file" '/dev/stdin'