check pull requests for prettier formatting

[nathan-muir]

In my other PR (#1959) I noticed there were some unrelated changes, because of formatting (trailing whitespaces etc).

If it helps, I've prepared this PR to:

• Add a github workflow to check PRs have applied prettier (via npm run fmt)
• Fixes some HTML errors that prevented prettier from running
• Applies prettier to all files

This does add a bit of code churn because of the addition of trailing commas, which could be disabled in prettier -- https://prettier.io/docs/options#trailing-commas

If you're concerned about this affecting git blame, after merging we can add a .git-blame-ignore-revs with the commit hash

[lavrton]

Helpful. But I think we need disable trailing commas

[nathan-muir]

es5 was the prettier default before prettier v3 (now all) [1] This results in much less code churn.

[1] https://prettier.io/docs/options#trailing-commas

[nathan-muir]

Helpful. But I think we need disable trailing commas

Ok, I've set to trailingComma: 'es5' which was prettier's default value before v3. Instead of 97 files modified, there are only 34.