Support built-in Zed prompts for all platforms (#26201)

This pull request does two things:

1. Adds a setting to force Zed to use the built-in prompts, instead of
the system provided ones. I've personally found the system prompts on
macOS often fail to respond to keyboard input, are slow to render
initially, and don't match Zed's style.
2. Makes the previously Linux-only Zed provided prompts available to
everybody using the above setting.

Release Notes:
- Added support for a built-in prompting system, regardless of platform.
Use the new `use_system_prompts` setting to control whether to use the
system provided prompts or Zed's built-in system. Note that on Linux,
this setting has no effect, as Linux doesn't have a system prompting
mechanism.

crates/workspace/src/workspace_settings.rs

@@ -19,6 +19,7 @@ pub struct WorkspaceSettings {
     pub restore_on_startup: RestoreOnStartupBehavior,
     pub drop_target_size: f32,
     pub use_system_path_prompts: bool,
+    pub use_system_prompts: bool,
     pub command_aliases: HashMap<String, String>,
     pub show_user_picture: bool,
     pub max_tabs: Option<NonZeroUsize>,
@@ -147,6 +148,13 @@ pub struct WorkspaceSettingsContent {
     ///
     /// Default: true
     pub use_system_path_prompts: Option<bool>,
+    /// Whether to use the system provided prompts.
+    /// When set to false, Zed will use the built-in prompts.
+    /// Note that this setting has no effect on Linux, where Zed will always
+    /// use the built-in prompts.
+    ///
+    /// Default: true
+    pub use_system_prompts: Option<bool>,
     /// Aliases for the command palette. When you type a key in this map,
     /// it will be assumed to equal the value.
     ///