x11-fonts/nerd-fonts: split into subports

Split the port into subports, each of which installs one group of Nerd
Fonts. The original port becomes the MASTERDIR and also a meta-port for
the subports. This prevents breakage for existing users/consumers.

This change has the following advantages over the previous monolithic
port of the Nerd Fonts:

* Users/consumers can install only the fonts they actually want to use.
  This reduces the required disc space tremendously.

* The licence for each group is properly documented and extracted into
  the expected path, i.e. ${PREFIX}/share/licenses/nerd-fonts/*/.

* Upstream provides release archives with stable checksums for each
  group. This is the preferred method for retrieving the distfiles.

While we are at it, assign maintainership to submitter due to multiple
consecutive maintainer timeouts.

PR:		284121
Reported by:	Denis Shaposhnikov <dsh@bamus.cz>
Approved by:	gregf@hugops.pw (maintainer timeout, 2 weeks)