[Proposal]: Relax ordering constraints around ref and partial modifiers on type declarations

[RikkiGibson]

Discussion: #8965

Currently, partial must appear directly before struct, class, or another type declaration keyword. If the type is a ref struct, ref must appear immediately before partial or struct. It seems likely that various other keywords could be used to disambiguate these contextual modifiers and allow us to relax the constraints on where partial and ref can appear in the modifier list.

See https://github.com/dotnet/csharplang/blob/master/meetings/2017/LDM-2017-12-04.md
See also #8134