settings_ui: Get editor settings working (#37330)

Closes #ISSUE

This PR includes the necessary work to get `EditorSettings` showing up
in the settings UI. Including making the `path` field on
`SettingsUiItem`'s optional so that top level items such as
`EditorSettings` which have `Settings::KEY = None` (i.e. are treated
like `serde(flatten)`) have their paths computed correctly for JSON
reading/updating.

It includes the first examples of a pattern I expect to continue with
the `SettingsUi` work with respect to settings reorganization, that
being adding missing defaults, and adding explicit values (or aliases)
to settings which previously relied on `null` being a value for optional
fields.

Release Notes:

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

crates/project/src/project_settings.rs

@@ -281,7 +281,17 @@ impl Default for GlobalLspSettings {
 }
 
 #[derive(
-    Clone, Copy, Debug, Eq, PartialEq, Ord, PartialOrd, Serialize, Deserialize, JsonSchema,
+    Clone,
+    Copy,
+    Debug,
+    Eq,
+    PartialEq,
+    Ord,
+    PartialOrd,
+    Serialize,
+    Deserialize,
+    JsonSchema,
+    SettingsUi,
 )]
 #[serde(rename_all = "snake_case")]
 pub enum DiagnosticSeverity {
@@ -290,6 +300,7 @@ pub enum DiagnosticSeverity {
     Error,
     Warning,
     Info,
+    #[serde(alias = "all")]
     Hint,
 }