Allow defining slash commands in extensions (#12255)

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

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

```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:

```rs
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

crates/extension_api/wit/since_v0.0.7/slash-command.wit

@@ -0,0 +1,11 @@
+interface slash-command {
+    /// A slash command for use in the Assistant.
+    record slash-command {
+        /// The name of the slash command.
+        name: string,
+        /// The description of the slash command.
+        description: string,
+        /// Whether this slash command requires an argument.
+        requires-argument: bool,
+    }
+}