.gitignore: add clangd-related files

I started using clangd as my LSP for C-based codebases and it works
quite well! Setting it up requires creating a `compile_commands.json`
file using e.g. `bear` (https://github.com/rizsotto/Bear).

Add it to the list of files to ignore. clangd caches its indexing in the
`.cache` directory. Add that too.

.gitignore

@@ -106,3 +106,7 @@
 /tests/check/test-lib-introspection.sh.trs
 /tests/kola
 _libs
+
+# for clangd: https://clangd.llvm.org/installation#project-setup
+/compile_commands.json
+/.cache