Install brush

If you don’t already have Rust + cargo installed, you’ll need to install them first.

We recommend using rustup.
Build + install brush:
```
cargo install --locked brush-shell
```
Make sure ~/.cargo/bin is in your PATH. If you used rustup to install Rust, then this will require ensuring that something like the following has been added to your your default shell’s rc file (e.g., .bashrc):
```
. "$HOME/.cargo/env"
```
Run brush:
```
brush
```