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

extensions/gleam/Cargo.toml

@@ -13,4 +13,4 @@ path = "src/gleam.rs"
 crate-type = ["cdylib"]
 
 [dependencies]
-zed_extension_api = "0.0.6"
+zed_extension_api = { path = "../../crates/extension_api" }

extensions/gleam/extension.toml

@@ -13,3 +13,7 @@ language = "Gleam"
 [grammars.gleam]
 repository = "https://github.com/gleam-lang/tree-sitter-gleam"
 commit = "8432ffe32ccd360534837256747beb5b1c82fca1"
+
+[slash_commands.gleam-project]
+description = "Returns information about the current Gleam project."
+requires_argument = false

extensions/gleam/src/gleam.rs

@@ -1,6 +1,6 @@
 use std::fs;
 use zed::lsp::CompletionKind;
-use zed::{CodeLabel, CodeLabelSpan, LanguageServerId};
+use zed::{CodeLabel, CodeLabelSpan, LanguageServerId, SlashCommand};
 use zed_extension_api::{self as zed, Result};
 
 struct GleamExtension {
@@ -142,6 +142,28 @@ impl zed::Extension for GleamExtension {
             code,
         })
     }
+
+    fn run_slash_command(
+        &self,
+        command: SlashCommand,
+        _argument: Option<String>,
+        worktree: &zed::Worktree,
+    ) -> Result<Option<String>, String> {
+        match command.name.as_str() {
+            "gleam-project" => {
+                let mut message = String::new();
+                message.push_str("You are in a Gleam project.\n");
+
+                if let Some(gleam_toml) = worktree.read_text_file("gleam.toml").ok() {
+                    message.push_str("The `gleam.toml` is as follows:\n");
+                    message.push_str(&gleam_toml);
+                }
+
+                Ok(Some(message))
+            }
+            command => Err(format!("unknown slash command: \"{command}\"")),
+        }
+    }
 }
 
 zed::register_extension!(GleamExtension);