Consider not requiring RescriptCore to be opened globally

[reebalazs]

Is your feature request related to a problem? Please describe.
I have a project that still uses Belt. By opening Belt when RescriptCore is open, there are a lot of shadow warnings that I want to get rid of.

Sure there is a solution, I can use open! Belt everywhere in my codebase to get rid of the warnings, but this is also not perfect, as now both libraries are open, one shadowing the other.

Describe the solution you'd like
Using RescriptCore.Option instead of Option in the generated code would be easy, and then opening RescriptCore globally would not be required.

Additional context
While I like RescriptCore and I think it's the possible future, it is better to leave it up to the developers whether or not they want to open it globally in their project, and let pgtyped-rescript work either way.