[#26] Add stylish-haskell and hlint

[dcastro]

Description

This PR completes the project setup process:

• Adds stylish-haskell, hlint and .editorconfig config files
• Applies stylish-haskell and hlint to the repo
• Adds build steps to the pipeline to verify that the repo adheres to stylish-haskell and hlint
• Adds a missing CONTRIBUTING.md and fixes the PR template file
• Applies and enforces ImportQualifiedPost via stylish-haskell checks

Since this PR touches many files, it's best to review it by reviewing one commit at a time (rather than everything at once).

Related issue(s)

Fixes part of #26

:white_check_mark: Checklist for your Pull Request

Related changes (conditional)

• Tests

  • [x] If I added new functionality, I added tests covering it.
  • [x] If I fixed a bug, I added a regression test to prevent the bug from silently reappearing again.

• Documentation

  • [x] I checked whether I should update the docs and did so if necessary:
  • README
  • Haddock

• Public contracts

  • [ ] Any modifications of public contracts comply with the Evolution of Public Contracts policy.
  • [ ] I added an entry to the changelog if my changes are visible to the users and
  • [ ] provided a migration guide for breaking changes if possible

Stylistic guide (mandatory)

• [x] My commits comply with the policy used in Serokell.
• [x] My code complies with the style guide.