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/src/wasm_host/wit.rs

@@ -19,7 +19,7 @@ use wasmtime::{
 pub use latest::CodeLabelSpanLiteral;
 pub use latest::{
     zed::extension::lsp::{Completion, CompletionKind, InsertTextFormat, Symbol, SymbolKind},
-    CodeLabel, CodeLabelSpan, Command, Range,
+    CodeLabel, CodeLabelSpan, Command, Range, SlashCommand,
 };
 pub use since_v0_0_4::LanguageServerConfig;
 
@@ -255,6 +255,22 @@ impl Extension {
             Extension::V001(_) | Extension::V004(_) => Ok(Ok(Vec::new())),
         }
     }
+
+    pub async fn call_run_slash_command(
+        &self,
+        store: &mut Store<WasmState>,
+        command: &SlashCommand,
+        argument: Option<&str>,
+        resource: Resource<Arc<dyn LspAdapterDelegate>>,
+    ) -> Result<Result<Option<String>, String>> {
+        match self {
+            Extension::V007(ext) => {
+                ext.call_run_slash_command(store, command, argument, resource)
+                    .await
+            }
+            Extension::V001(_) | Extension::V004(_) | Extension::V006(_) => Ok(Ok(None)),
+        }
+    }
 }
 
 trait ToWasmtimeResult<T> {