New package: uosc-5.12.0

[dogknowsnx]

https://github.com/tomasklaen/uosc

Testing the changes

• I tested the changes in this PR: YES
• been using uosc for years

New package

• This new package conforms to the package requirements: YES

Local build testing

• I built this PR locally for my native architecture, x86_64-glibc

Notes:

• In order to make this package work out-of-the-box (which I presume to be desired, when installing from a package manager), files are going to be installed into /etc/mpv, hence globally. Otherwise the user would have to copy everything manually from e.g. /usr/share, in which case I'd recommend using the installation script provided by upstream, which will copy everything needed into one's home directory.
  EDIT: I reconsidered this and opted for the (preferred) per-user installation, which requires additional steps (see README.voidlinux), but is more in line w/ Void's packaging guidelines.

• There is a utility binary called "ziggy" that can be built separately (go language), which is mainly used for updating uosc locally, but it also implements clipboard support, extra subtitle downloads and maybe some other features. I'm not interested in shipping said binary and bloating the package unnecessarily, since lua-uosc (which advertises itself as being "minimalist") does everything I need from a UI for mpv.