Add swiftformat linter

[ShadowJonathan]

On a related note, I was planning to bring swiftformat into the project when I get a moment, likely set up as a pre-commit hook, what are your thoughts?

The thinking is it'll ensure the project is formatted to the "official" language style, and saves any style based stuff creeping into reviews?

Originally posted by @mormaer in https://github.com/buresdv/Mlem/pull/145#discussion_r1229878829