Add clangd cache to .gitignore (LMMS#7846)

* Add /.cache/ to .gitignore

Normally clangd's .cache exists in /build/, which is already in the .gitignore, but if cmake is run in the repository root instead of /build/ the cache is in turn generated in the repository root.

* Also add compile_commands.json to .gitignore

Author: rubiefawn

.gitignore

@@ -10,3 +10,5 @@
 /plugins/ZynAddSubFx/zynaddsubfx/doc/gen/Makefile
 /data/locale/*.qm
 Brewfile.lock.json
+/.cache/
+compile_commands.json