feat: clangd support

[alphabitserial]

This adds:

• a configuration file for clangd (to ignore flags clang doesn't support)
• .gitignore entry for clangd's cache directory

It also adds compile_commands.json to .gitignore because it must be generated by the user themselves. This is because it requires hardcoded paths. I've also opened a PR on the Star Haven docs with instructions for generating this file: https://github.com/star-haven/docs/pull/5

[bates64]

This is great to see, thanks for these PRs! This change would suit pmret/papermario. Could you submit this patch to upstream instead?

[alphabitserial]

You got it!