extension: Make worktree argument to run_slash_command optional (#15658)

This PR updates the extension API to make the `worktree` argument to
`run_slash_command` optional.

We may not always have a worktree, and not all slash commands need them,
so by making it optional we can allow individual slash commands to
decide what to do when there is no worktree.

Release Notes:

- N/A

extensions/gleam/src/gleam.rs

@@ -181,7 +181,7 @@ impl zed::Extension for GleamExtension {
         &self,
         command: SlashCommand,
         argument: Option<String>,
-        worktree: &zed::Worktree,
+        worktree: Option<&zed::Worktree>,
     ) -> Result<SlashCommandOutput, String> {
         match command.name.as_str() {
             "gleam-docs" => {
@@ -218,6 +218,8 @@ impl zed::Extension for GleamExtension {
                 })
             }
             "gleam-project" => {
+                let worktree = worktree.ok_or_else(|| "no worktree")?;
+
                 let mut text = String::new();
                 text.push_str("You are in a Gleam project.\n");