Add example extension to showcase slash commands (#16300)

This PR adds an example extension to showcase how to write slash
commands in an extension.

Release Notes:

- N/A

extensions/slash-commands-example/src/slash_commands_example.rs

@@ -0,0 +1,90 @@
+use zed_extension_api::{
+    self as zed, SlashCommand, SlashCommandArgumentCompletion, SlashCommandOutput,
+    SlashCommandOutputSection, Worktree,
+};
+
+struct SlashCommandsExampleExtension;
+
+impl zed::Extension for SlashCommandsExampleExtension {
+    fn new() -> Self {
+        SlashCommandsExampleExtension
+    }
+
+    fn complete_slash_command_argument(
+        &self,
+        command: SlashCommand,
+        _args: Vec<String>,
+    ) -> Result<Vec<zed_extension_api::SlashCommandArgumentCompletion>, String> {
+        match command.name.as_str() {
+            "echo" => Ok(vec![]),
+            "pick-one" => Ok(vec![
+                SlashCommandArgumentCompletion {
+                    label: "Option One".to_string(),
+                    new_text: "option-1".to_string(),
+                    run_command: true,
+                },
+                SlashCommandArgumentCompletion {
+                    label: "Option Two".to_string(),
+                    new_text: "option-2".to_string(),
+                    run_command: true,
+                },
+                SlashCommandArgumentCompletion {
+                    label: "Option Three".to_string(),
+                    new_text: "option-3".to_string(),
+                    run_command: true,
+                },
+            ]),
+            command => Err(format!("unknown slash command: \"{command}\"")),
+        }
+    }
+
+    fn run_slash_command(
+        &self,
+        command: SlashCommand,
+        args: Vec<String>,
+        _worktree: Option<&Worktree>,
+    ) -> Result<SlashCommandOutput, String> {
+        match command.name.as_str() {
+            "echo" => {
+                if args.is_empty() {
+                    return Err("nothing to echo".to_string());
+                }
+
+                let text = args.join(" ");
+
+                Ok(SlashCommandOutput {
+                    sections: vec![SlashCommandOutputSection {
+                        range: (0..text.len()).into(),
+                        label: "Echo".to_string(),
+                    }],
+                    text,
+                })
+            }
+            "pick-one" => {
+                let Some(selection) = args.first() else {
+                    return Err("no option selected".to_string());
+                };
+
+                match selection.as_str() {
+                    "option-1" | "option-2" | "option-3" => {}
+                    invalid_option => {
+                        return Err(format!("{invalid_option} is not a valid option"));
+                    }
+                }
+
+                let text = format!("You chose {selection}.");
+
+                Ok(SlashCommandOutput {
+                    sections: vec![SlashCommandOutputSection {
+                        range: (0..text.len()).into(),
+                        label: format!("Pick One: {selection}"),
+                    }],
+                    text,
+                })
+            }
+            command => Err(format!("unknown slash command: \"{command}\"")),
+        }
+    }
+}
+
+zed::register_extension!(SlashCommandsExampleExtension);