Should be able to modify MergedConfigurationProperties in TinyMceOptions

Description

In CMS 11, the MergedConfigurationProperties is an IEnumerable, meaning that it is not possible to add additional properties to this collection. It should be possible to configure this collection as was the case in CMS 10.

The IEnumerable should be changed to IList. It won't be possible to override properties, but it will be possible to add and remove items.

context.Services.Configure<EPiServer.Cms.TinyMce.Core.TinyMceOptions>(config =>
{
    config.MergedConfigurationProperties.Remove("invalid_elements");
    config.MergedConfigurationProperties.RemoveAt(2);
    config.MergedConfigurationProperties.RemoveAt("test");
}