Clones the Ghostty source, builds libghostty-vt, and installs the
shared library and headers into priv/.
This only needs to run once. After that, GHOSTTY_BUILD=1 mix compile
will find the library in priv/ and build the NIF from source.
Set GHOSTTY_SOURCE_DIR to skip cloning and use a local checkout.
Examples
mix ghostty.setup
GHOSTTY_SOURCE_DIR=~/code/ghostty mix ghostty.setup