Adding Sheldon to benchmark

[masterberg]

Hi, there is a new plugin manager called Sheldon, it was made in rust and might bring some value as it's getting some attention recently.
I'll try to PR a basic setup in a while.

[romkatv]

Cool. Do send a PR. In the meantime I sent them a PR: rossmacarthur/zsh-plugin-manager-benchmark#13.

Edit: It appears that zsh4humans beats all plugin managers on "load" and "install" benchmarks, even though zsh4humans does a log of stuff in addition to managing plugins. I'll wait for rossmacarthur to publish the official numbers though.