chore(gitignore): add .jj/

.gitignore

@@ -1,3 +1,11 @@
+# Jujutsu
+#
+# When using the WGPU repository through [Jujutsu](https://github.com/martinvonz/jj), `.jj/` dirs.
+# are ignored because Jujutsu writes a `.jj/.gitignore` file containing `/*`. Some tools, like
+# `prettier`, don't handle nested `.gitgnore` properly, but they _do_ handle top-level `.gitignore`
+# patterns properly. So, include it here, even though we shouldn't need it. :cry:
+.jj/
+
 # Generated by Cargo
 # will have compiled files and executables
 target/