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/extensions_ui/src/extensions_ui.rs

@@ -1345,7 +1345,7 @@ impl ExtensionsPage {
                             this.update_settings::<VimModeSetting>(
                                 selection,
                                 cx,
-                                |setting, value| *setting = Some(value),
+                                |setting, value| setting.vim_mode = Some(value),
                             );
                         }),
                     )),