Adds Swiftlint formatter

[jakenvac]

I've added swiftlint as a formatter, I've tested over the weekend and it works great.

The command used to format is as follows:

swiftlint lint \
  --use-stdin \ # take input from stdin
  --fix \ # automatically fix errors where possible
  --format # apply sourcekit lsp default formatting if available

[stevearc]

Thanks for the PR!