Handle `disable_plugin_host_3.3` setting

[FichteFoll]

1. This setting name gets matched by our package namespace highlighting and needs to be circumvented, e.g. by using a negative look-ahead like \.(?!\d).
   
2. The setting is undocumented, so we need to add metadata for it.

[FichteFoll]

Reopening because the metadata has not been added yet.