feat: github action to run prettier

[pkra]

Runs pretter on PRs to the main branch;
keeping to the defaults for now.

[pkra]

@jnurthen @spectranaut This feels simpler to me than #49, which only reports errors and expects people to fix their PRs to pass linting rules they might not know much about.

What do you think?

[spectranaut]

I do really like this, a lot. Do you want to add this to all the spec repos directly? Is the PR here just demonstration, because aren't we working towards retiring this repo?

[pkra]

As per the last editors meeting, we want to try this approach for other repositories (but not here).