summary:
- adds prettier formatter configuration
- formats the entire repo using prettier, ignoring minified files
(`*.min.css`) and heavy generated html
- changes extensions of all `.html` files to `.liquid`, which is more
correct and necessary for prettier to work correctly
- replaces "%-" and "-%" with just "%" — manual liquid formatting using
minus signs is superfluous since we are compressing and minifying the
code anyway
- adds CI action for running prettier check on PR and pushes to master
