settings ui: Create settings key trait (#37489)

This PR separates out the associated constant `KEY` from the `Settings`
trait into a new trait `SettingsKey`. This allows for the key trait to
be derived using attributes to specify the path so that the new
`SettingsUi` derive macro can use the same attributes to determine top
level settings paths thereby removing the need to duplicate the path in
both `Settings::KEY` and `#[settings_ui(path = "...")]`

Co-authored-by: Ben Kunkle <ben@zed.dev>

Release Notes:

- N/A

---------

Co-authored-by: Ben Kunkle <ben@zed.dev>

crates/agent_ui/src/slash_command_settings.rs

@@ -2,10 +2,11 @@ use anyhow::Result;
 use gpui::App;
 use schemars::JsonSchema;
 use serde::{Deserialize, Serialize};
-use settings::{Settings, SettingsSources, SettingsUi};
+use settings::{Settings, SettingsKey, SettingsSources, SettingsUi};
 
 /// Settings for slash commands.
-#[derive(Deserialize, Serialize, Debug, Default, Clone, JsonSchema, SettingsUi)]
+#[derive(Deserialize, Serialize, Debug, Default, Clone, JsonSchema, SettingsUi, SettingsKey)]
+#[settings_key(key = "slash_commands")]
 pub struct SlashCommandSettings {
     /// Settings for the `/cargo-workspace` slash command.
     #[serde(default)]
@@ -21,8 +22,6 @@ pub struct CargoWorkspaceCommandSettings {
 }
 
 impl Settings for SlashCommandSettings {
-    const KEY: Option<&'static str> = Some("slash_commands");
-
     type FileContent = Self;
 
     fn load(sources: SettingsSources<Self::FileContent>, _cx: &mut App) -> Result<Self> {