Add a new interface allowing to set a default dialect to be used for printing/parsing regions

Currently the builtin dialect is the default namespace used for parsing
and printing. As such module and func don't need to be prefixed.
In the case of some dialects that defines new regions for their own
purpose (like SpirV modules for example), it can be beneficial to
change the default dialect in order to improve readability.

Differential Revision: https://reviews.llvm.org/D107236