Add a schema for the markup FieldGroupFormatter plugin

Problem/Motivation

When running a kernel test for a view display with the markup plugin, the test currently fails with a SchemaIncompleteException error. This is caused by the fact that the module doesn't provide a schema for the plugin.

Proposed resolution

Add a field_group_markup.field_group_formatter_plugin.schema.yml file with the correct schema for the plugin.

Remaining tasks

1. Write a patch
2. Review
3. Commit

User interface changes

None

API changes

None

Data model changes

None