Add check for missing Stable Tag in readme

[ernilambar]

• Update function check_stable_tag() to test if Stable Tag is missing
• Raise ERROR for missing stable tag
• Update tests to reflect new changes

Since the value of Stable Tag is very important in our directory, I have kept this check as ERROR. We can keep it as WARNING if otherwise. This check is currently not there in legacy version.

cc: @swissspidy @felixarntz @mukeshpanchal27

[swissspidy]

@foosantos @bordoni Is such a check for missing Stable Tag useful for the plugin review team?

[foosantos]

Yes, that is indeed one of the things that is checked manually, so surely relevant. Thank you, @ernilambar and @swissspidy!