Set up prettier

[ds300]

I noticed this repo has prettier installed but not set up on CI so the file formatting is quite inconsistent. This makes it hard to work on if you are one of the many people who have prettier integrated in their editor/IDE because it keeps introducing unrelated formatting changes.

This PR

• Configures prettier with angular's config since the signals code was originally forked from there. Happy to change this based on maintainer's preferences.
• Runs prettier to bring all files up to date.
• Adds a CI check to make sure the formatting remains consistent over time

[NullVoxPopuli]

Thank you!