WIP: Remove SDocs from ErrCtxt & ErrInfo

This MR:

• turns the SDoc used in ErrCtxt into a proper error datatype, ErrCtxtMsg, which contains all the different error contexts that can be added,

• removes the SDocs in ErrInfo. The first SDoc is changed to [ErrCtxt], while the second is Maybe ErrInfoSupplementary, where ErrInfoSupplementary is a new datatype that only has two constructors:

  • displaying extra context in a constraint solver message, such as hole fits and relevant constraints,

  • displaying name suggestions for an out of scope error.