CI: Add simple code linting workflow

[guihkx]

Right now it only does two things:

• Run clang-format on first-party sources (and returns an error if it finds any unformatted file)
• Run cmake-format on CMakeLists.txt (and returns an error if the file is not properly formatted)

In the future, we can use this workflow to add more helpful tools, like clang-tidy, to help us keep the code base consistent.

Ah, I obviously had to format all files in src/, otherwise the CI would fail. :)

[nuttyartist]

Nicee 👍