Add plugin for rustup completion (#8914)

plugins/rustup/README.md

+# rustup
+
+This plugin adds completion for [`rustup`](https://rustup.rs/), the toolchain installer for the Rust programming language.
+
+To use it, add `rustup` to the plugins array in your zshrc file:
+
+```zsh
+plugins=(... rustup)
+```