Add `check_command_available` functionality for basic-cli

[lukewilliamboswell]

Here is an example rust implementation from roc-lang/roc/crates/utils/command/src/lib.rs.

This will be useful for scripts to check a command exists before trying to run it.

fn check_command_available(command_name: &str) -> bool {
    if cfg!(target_family = "unix") {
        let unparsed_path = match std::env::var("PATH") {
            Ok(var) => var,
            Err(VarError::NotPresent) => return false,
            Err(VarError::NotUnicode(_)) => {
                panic!("found PATH, but it included invalid unicode data!")
            }
        };

        std::env::split_paths(&unparsed_path).any(|dir| dir.join(command_name).exists())
    } else if cfg!(target = "windows") {
        let mut cmd = Command::new("Get-Command");

        cmd.args([command_name]);

        let cmd_str = format!("{cmd:?}");

        let cmd_out = cmd.output().unwrap_or_else(|err| {
            panic!(
                "Failed to execute `{cmd_str}` to check if {command_name} is available:\n    {err}"
            )
        });

        cmd_out.status.success()
    } else {
        // We're in uncharted waters, best not to panic if
        // things may end up working out down the line.
        true
    }
}