feature: publish on AUR (grrs-git and grrs)

set up PKGBUILDs to distribute grrs to archlinux users via AUR. will need to write a PKGBUILD for grrs-git that pulls source directly from radicle seed. also need to write a PKGbuild for the grrs package that builds from the crates.io tarball. if i ever wanna do a grrs-bin it'll be necessary to host the pre-compiled binary somewhere. need to ensure that both PKGBUILDs correctly install the binary + planned .desktop file (see issue a9a668b). both packages will also need .SRCINFO files, and need to be tested locally using makepkg.