zsh: Add yarn global bin directory to PATH
zsh: Add yarn global bin directory to PATH

Yarn installs here by default. We can override on a per install basis,
but I expect I'd forget from time to time and would rather the default
install location just work.

I'm hard coding the path, rather than dynamically querying with `$(yarn
global bin)` as the dynamic query adds 0.17s to zsh startup time and
that is farrrrrr too much.

christoomey/dotfiles,christoomey/dotfiles