Add reproducible performance tests comparing zinit and other plugin managers

[alichtman]

One of the main selling points of zinit is that it's fast. Freakishly fast. Let's provide a quantifiable look into how fast it is. The chart in the README is old + probably outdated.

This is probably best thrown in some container that you can run on your own machine? No need to spend CI cycles on this.

We should set up identical configs for Oh-My-Zsh, zplug, and zinit and track "time to usable shell".

[pschmitt]

https://github.com/romkatv/zsh-bench