feat(Util): add formally solved here option to the category tag

[Paul-Lez]

As discussed here

[mo271]

thanks, that looks great already, will look at it more closely later.

somewhat related, currently we aren't checking against

@[category research open, AMS 11, category research solved]

right?
Perhaps we can somehow ensure that exactly one category is provided. With the new one it might be conceivable that people try to apply both solved and formallly solved here?

[Paul-Lez]

Opened an issue for this #1852

[mo271]

Thanks, looks great!