[Enhancement] Move types for "Extras" outside of main types.d.ts

[jonobr1]

Is your feature request related to a problem? Please describe.
The main types.d.ts currently has types for Two.ZUI, but not types for Two.Arc. This is because adding them creates conflicts for the documentation with Two.ArcSegment. Also, perhaps there isn't a need to bloat the types.d.ts file with information that developers may not use.

Describe the solution you'd like
Create a system / rule for defining types of modules in the /extras folder.

Additional context
See how other libraries handle this, like Three.js