Add a per-user editor settings service for server-readable preferences

Problem/Motivation

Server-readable editor preferences (hotkeys, media file associations, tool toggles) are stored per-account on the user's navigation_plus_settings map field, but each new preference needed its own bespoke save route and controller. There was no single read/write API, which made adding a preference more work than it should be.

Proposed resolution

• Add an EditorSettings service exposing get($namespace, $key, $default) and set($namespace, $key, $value), namespaced by the owning module and persisted on the live user entity (outside any workspace).
• Add a generic /navigation-plus/setting/{namespace}/{key}/{value} route to Settings::saveSetting(), which calls EditorSettings::set().
• Add a Drupal.NavigationPlus.saveSetting(namespace, key, value) JS helper as the client counterpart.

Purely visual, client-only toggles (block preview, layout outlines) remain in the browser's localStorage and are intentionally not handled
here.

API changes

• New navigation_plus.editor_settings service (EditorSettings).
• New route navigation_plus.settings.save_setting.
• New JS helper Drupal.NavigationPlus.saveSetting().