Groups config in mercury_editor.menu.settings should be YAML instead of string

Problem/Motivation

When making schema updates for 🐛 Schema errors for mercury_editor.settings Active , I discovered that the configuration for groups in mercury_editor.menu.settings is in string format, rather than YAML. This doesn't cause a problem, but it does prevent more detailed validation of the schema.

Proposed resolution

Update the schema and MenuSettingsForm.php to handle YAML, as is done in DialogSettingsForm.php. Create an update hook to capture configuration in existing sites that is stored as a string.