Add constants for default options in DiffXWriter.
Review Request #11728 — Created July 17, 2021 and submitted
These default options are used when writing a DiffX file. The options
were previously stored as hard-coded values in function definitions, but
this meant that other parts of the codebase couldn't refer to those
It also adds a new options definition,
MetaFormat, for available
metadata formats. This just includes
Unit tests passed.
Used this with the upcoming DOM change.
Mind having one arg per line instead of mixing it like this?