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 ...

assets/settings/default.json

@@ -188,8 +188,8 @@
   //  4. A box drawn around the following character
   //     "hollow"
   //
-  // Default: not set, defaults to "bar"
-  "cursor_shape": null,
+  // Default: "bar"
+  "cursor_shape": "bar",
   // Determines when the mouse cursor should be hidden in an editor or input box.
   //
   // 1. Never hide the mouse cursor:
@@ -282,8 +282,8 @@
   //  - "warning"
   //  - "info"
   //  - "hint"
-  //  - null — allow all diagnostics (default)
-  "diagnostics_max_severity": null,
+  //  - "all" — allow all diagnostics (default)
+  "diagnostics_max_severity": "all",
   // Whether to show wrap guides (vertical rulers) in the editor.
   // Setting this to true will show a guide at the 'preferred_line_length' value
   // if 'soft_wrap' is set to 'preferred_line_length', and will show any