Allow defining slash commands in extensions

[maxdeviant]

This PR adds initial support for defining slash commands for the Assistant from extensions.

Slash commands are defined in an extension's extension.toml:

[slash_commands.gleam-project]
description = "Returns information about the current Gleam project."
requires_argument = false

and then executed via the run_slash_command method on the Extension trait:

impl Extension for GleamExtension {
    // ...

    fn run_slash_command(
        &self,
        command: SlashCommand,
        _argument: Option<String>,
        worktree: &zed::Worktree,
    ) -> Result<Option<String>, String> {
        match command.name.as_str() {
            "gleam-project" => Ok(Some("Yayyy".to_string())),
            command => Err(format!("unknown slash command: \"{command}\"")),
        }
    }
}

Release Notes:

• N/A