feat: add pre-commit hooks

[danthorpe]

I took a shot at adding a pre-commit hooks files, according to this guide.

Fixes #676.

---

Once this is merged, it can be used in your other swift packages, by adding the following to the .pre-commit-config.yaml under the repos section, according to the guide.

  - repo: https://github.com/apple/swift-format
    rev: <commit hash / tag>
    hooks:
      - id: swift-format

[Feuermurmel]

I'm test-driving the hook from this MR in one of my projects using this configuration:

repos:
  - repo: https://github.com/danthorpe/swift-format
    rev: danthorpe/add_pre_commit_hooks
    hooks:
      - id: swift-format

There are two things I noticed:

• I think the hook should have require_serial: true set, because swift-format already processes the input files in parallel.

  When require_serial is set to true, only a single process is started for the hook, with all file names passed to it.

• IMHO it would be nice if swift-format printed the names of files that have been changed. It gives a bit more context as to why the hook failed as well as how many files have been changed.

  I'm mainly a Python dev and I'm using black and isort in most of my projects. Compare their output to that of black:

  

  AFACIT, swift-format has no option to enable that right now.

In any case, happy to see this moving forward! 😊

[danthorpe]

Thanks for the comments @Feuermurmel - I wasn't familiar with the require_serial option, I'll take a look into and update the branch/PR.