Create .prettierrc.json

[hiimpoop]

Add Prettier configuration

[github-actions]

Hi @hiimpoop,

Thank you for your interest in contributing to Malachite.

This PR has been automatically closed because it does not reference a GitHub issue. All PRs must reference an existing issue using the format Closes: #XXX.

To contribute properly:

1. Find an existing issue you'd like to work on, or open a new issue describing your proposed change
2. Comment on the issue requesting assignment and wait for maintainer approval
3. Only submit a PR after you have been assigned to the issue

Please see our CONTRIBUTING.md for more details.