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/settings/src/base_keymap_setting.rs

@@ -1,10 +1,10 @@
 use std::fmt::{Display, Formatter};
 
-use crate as settings;
+use crate::{self as settings};
 use schemars::JsonSchema;
 use serde::{Deserialize, Serialize};
 use settings::{Settings, SettingsSources, VsCodeSettings};
-use settings_ui_macros::SettingsUi;
+use settings_ui_macros::{SettingsKey, SettingsUi};
 
 /// Base key bindings scheme. Base keymaps can be overridden with user keymaps.
 ///
@@ -101,16 +101,25 @@ impl BaseKeymap {
 }
 
 #[derive(
-    Copy, Clone, Debug, Serialize, Deserialize, JsonSchema, PartialEq, Eq, Default, SettingsUi,
+    Copy,
+    Clone,
+    Debug,
+    Serialize,
+    Deserialize,
+    JsonSchema,
+    PartialEq,
+    Eq,
+    Default,
+    SettingsUi,
+    SettingsKey,
 )]
 // extracted so that it can be an option, and still work with derive(SettingsUi)
+#[settings_key(None)]
 pub struct BaseKeymapSetting {
     pub base_keymap: Option<BaseKeymap>,
 }
 
 impl Settings for BaseKeymap {
-    const KEY: Option<&'static str> = None;
-
     type FileContent = BaseKeymapSetting;
 
     fn load(