add prettier as a precommit hook

[samanpwbb]

Mapbox frontend projects have been standardizing around prettier for automated code formatting. This project would benefit prettier as it's code style is not consistent.

@scottsfarley93 / @katydecorah any objections to adding a prettier hook here?

[scottsfarley93]

👍 sounds good!

[samanpwbb]

Prettier migrations can cause annoying conflicts, so best to wait for the current wave of PRs to merge before implementing this.

[katydecorah]

No objections! 🙏