settings_ui: Add maybe settings (#40724)

Closes #ISSUE

Adds a `Maybe<T>` type to `settings_content`, that makes the distinction
between `null` and omitted settings values explicit. This unlocks a few
more settings in the settings UI

Release Notes:

- N/A *or* Added/Fixed/Improved ...

crates/settings/src/vscode_import.rs

@@ -853,7 +853,7 @@ impl VsCodeSettings {
 
     fn worktree_settings_content(&self) -> WorktreeSettingsContent {
         WorktreeSettingsContent {
-            project_name: None,
+            project_name: crate::Maybe::Unset,
             file_scan_exclusions: self
                 .read_value("files.watcherExclude")
                 .and_then(|v| v.as_array())