You should not be specifying things like this in your custom node. these kind of settings are personal to a user and should be user determined by the options object in settings.js or the users installed theme.
As for adding modules to monaco editor, this is not something yet formalised or implemented in the core as an exposed API. You could look at the node-red source and see if you can support this for the up-coming Node-RED v4 release (now is the time to add this kind of support)
I need this configuration for my custom node exclusively and for me, the important thing is to achieve adding modules. Having the editor instance, it is possible to access to the monaco instance?