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

Cargo.lock

@@ -436,6 +436,7 @@ dependencies = [
  "derive_more",
  "futures 0.3.28",
  "gpui",
+ "language",
  "parking_lot",
 ]
 
@@ -3784,6 +3785,7 @@ name = "extension"
 version = "0.1.0"
 dependencies = [
  "anyhow",
+ "assistant_slash_command",
  "async-compression",
  "async-tar",
  "async-trait",
@@ -13249,7 +13251,7 @@ dependencies = [
 name = "zed_gleam"
 version = "0.1.3"
 dependencies = [
- "zed_extension_api 0.0.6",
+ "zed_extension_api 0.0.7",
 ]
 
 [[package]]