Regression fix: leave R prefixes capitalization alone (#2285)

`black.strings.get_string_prefix` used to lowercase the extracted
prefix before returning it. This is wrong because 1) it ignores the
fact we should leave R prefixes alone because of MagicPython, and 2)
there is dedicated prefix casing handling code that fixes issue 1.
`.lower` is too naive.

This was originally fixed in 20.8b0, but was reintroduced since 21.4b0.

I also added proper prefix normalization for docstrings by using the
`black.strings.normalize_string_prefix` helper.

Some more test strings were added to make sure strings with capitalized
prefixes aren't treated differently (actually happened with my original
patch, Jelle had to point it out to me).