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/git_ui/src/git_panel_settings.rs

@@ -2,7 +2,7 @@ use editor::ShowScrollbar;
 use gpui::Pixels;
 use schemars::JsonSchema;
 use serde_derive::{Deserialize, Serialize};
-use settings::{Settings, SettingsSources, SettingsUi};
+use settings::{Settings, SettingsKey, SettingsSources, SettingsUi};
 use workspace::dock::DockPosition;
 
 #[derive(Copy, Clone, Debug, Serialize, Deserialize, JsonSchema, PartialEq, Eq)]
@@ -36,7 +36,8 @@ pub enum StatusStyle {
     LabelColor,
 }
 
-#[derive(Clone, Default, Serialize, Deserialize, JsonSchema, Debug, SettingsUi)]
+#[derive(Clone, Default, Serialize, Deserialize, JsonSchema, Debug, SettingsUi, SettingsKey)]
+#[settings_key(key = "git_panel")]
 pub struct GitPanelSettingsContent {
     /// Whether to show the panel button in the status bar.
     ///
@@ -90,8 +91,6 @@ pub struct GitPanelSettings {
 }
 
 impl Settings for GitPanelSettings {
-    const KEY: Option<&'static str> = Some("git_panel");
-
     type FileContent = GitPanelSettingsContent;
 
     fn load(