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/theme/src/settings.rs

@@ -13,7 +13,7 @@ use gpui::{
 use refineable::Refineable;
 use schemars::{JsonSchema, json_schema};
 use serde::{Deserialize, Serialize};
-use settings::{ParameterizedJsonSchema, Settings, SettingsSources, SettingsUi};
+use settings::{ParameterizedJsonSchema, Settings, SettingsKey, SettingsSources, SettingsUi};
 use std::sync::Arc;
 use util::ResultExt as _;
 use util::schemars::replace_subschema;
@@ -366,7 +366,8 @@ impl IconThemeSelection {
 }
 
 /// Settings for rendering text in UI and text buffers.
-#[derive(Clone, Debug, Default, Serialize, Deserialize, JsonSchema, SettingsUi)]
+#[derive(Clone, Debug, Default, Serialize, Deserialize, JsonSchema, SettingsUi, SettingsKey)]
+#[settings_key(None)]
 pub struct ThemeSettingsContent {
     /// The default font size for text in the UI.
     #[serde(default)]
@@ -818,8 +819,6 @@ fn clamp_font_weight(weight: f32) -> FontWeight {
 }
 
 impl settings::Settings for ThemeSettings {
-    const KEY: Option<&'static str> = None;
-
     type FileContent = ThemeSettingsContent;
 
     fn load(sources: SettingsSources<Self::FileContent>, cx: &mut App) -> Result<Self> {