New port: x11/alacritty

Alacritty is the fastest terminal emulator in existence. Using the
GPU for rendering enables optimizations that simply aren't possible in
other emulators.

Alacritty is focused on simplicity and performance. The performance
goal means it should be faster than any other terminal emulator
available. The simplicity goal means that it doesn't have many
features like tabs or scroll back as in other terminals. Instead, it
is expected that users of Alacritty make use of a terminal multiplexer
such as tmux.

WWW: https://github.com/jwilm/alacritty

Approved by: mat (mentor)
Differential Revision: https://reviews.freebsd.org/D11162