settings_ui: Add placeholder view (#15019)

This PR adds a placeholder view for the settings UI. It does not contain
any functionality, as of yet.

This view is staff-shipped behind a feature flag.

Release Notes:

- N/A

crates/zed/Cargo.toml

@@ -86,6 +86,7 @@ search.workspace = true
 serde.workspace = true
 serde_json.workspace = true
 settings.workspace = true
+settings_ui.workspace = true
 simplelog.workspace = true
 smol.workspace = true
 snippet_provider.workspace = true