Add GHA to check formatting

[gvrooyen]

This should probably be a separate GHA. The formatter should fail if something is found so the contributor can fix it themselves. Several tracks follow that setup. (@BNAndras)

Add a GitHub action that checks the following:

• Whether any source files have not been formatted. The current version of odinfmt only writes the correctly formatted file to stdout, or it overwrites the source file if the -w flag is specified. However, diff <(odinfmt $F | perl -pe 'chomp if eof') $F correctly returns a status code of 0 if a file is correctly formatted, or 1 if not. The GHA could check for formatting errors and advise the contributor to run odinfmt on the file (or bin/format-all.sh to fix all source files).
• Whether config files are correctly formatted (configlet fmt). This could prompt the contributor to run configlet fmt -u to update the offending files.