Custom importers shouls be able to easily add .plugin() statements to main.js

**I'm submitting a feature request**

One of the common use cases for custom importers is to add .plugin() statements to main.js. We could expose a service to custom importers that can do this. It is quite tricky to patch main.js due to the irregular format of main.js

Start by reading the custom importer flow and the irregular main.js format described in the issue. Determine how a service could let custom importers add .plugin() statements without directly patching main.js; done means the supported mechanism is exposed and custom importers can use it reliably.

Written by the indexing model from the issue text.