implement purge safelist

[RobinMalfait]

• fix --help output in tests
• add tests to ensure we can use purge.safelist
• implement the purge.safelist for strings

[chris-morgan]

I just happened to read the 2.2.0 release notes, and just want to say: safelist is a rather bad name here, honestly worse to my Australian eyes than whitelist, which would still be a poor name for the same reason: it doesn’t convey the meaning well. I recommend deprecating this newly-released name in favour of something semantically superior.

Some relevant words: “retain”, “keep”, “preserve”; as an outsider, I think my favourites from this couple of minutes of thought are keepClasses and retain.

(Feel free to do something or ignore me as you desire; I don’t intend to engage any further; I don’t even use Tailwind at this time.)

[xiCO2k]

@RobinMalfait Hey Robin, checking out the safelist stuff, there is any particular reason to not support an array with regex's?

Checked the documentation on the PurgeCSS docs (https://purgecss.com/safelisting.html#patterns) and does looks like is supported.

Thanks