[mlir][OpFormat] Add support for emitting newlines from the custom format of an operation

This revision adds a new `printNewline` hook to OpAsmPrinter that allows for printing a newline within the custom format of an operation, that is then indented to the start of the operation. Support for the declarative assembly format is also added, in the form of a `\n` literal.

Differential Revision: https://reviews.llvm.org/D93151