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/project/src/project.rs

@@ -11175,7 +11175,7 @@ impl<P: AsRef<Path>> From<(WorktreeId, P)> for ProjectPath {
     }
 }
 
-struct ProjectLspAdapterDelegate {
+pub struct ProjectLspAdapterDelegate {
     project: WeakModel<Project>,
     worktree: worktree::Snapshot,
     fs: Arc<dyn Fs>,
@@ -11185,7 +11185,11 @@ struct ProjectLspAdapterDelegate {
 }
 
 impl ProjectLspAdapterDelegate {
-    fn new(project: &Project, worktree: &Model<Worktree>, cx: &ModelContext<Project>) -> Arc<Self> {
+    pub fn new(
+        project: &Project,
+        worktree: &Model<Worktree>,
+        cx: &ModelContext<Project>,
+    ) -> Arc<Self> {
         Arc::new(Self {
             project: cx.weak_model(),
             worktree: worktree.read(cx).snapshot(),