This introduces `DiffXParser`, a new diff parser that can parse DiffX files, using the `pydiffx` module. SCMTools can provide this as the diff parser if they permit DiffX files to be used (whether DiffX is used natively, or if they look for a `#diffx:` header in `get_parser()`). This parser first turns DiffX file contents into a series of DOM (DiffX Object Model) objects, which are then turned into `ParsedDiff` objects. The `ParsedDiff` objects are populated with all the information needed to reconstruct the DiffX file perfectly (options, preambles, metadata, and diff contents). That reconstruction happens when downloading a raw diff. With normal diff parsers, that reconstruction just builds up a byte string containing the diff content for each file (which may include metadata or special lines for that diff variant). With `DiffXParser`, the content is instead built up using that parsed information stored in `extra_data`, along with the actual diff content (that in a `#...diff:` section). It's assembled back into a DOM, and then serialized. All the extracted data is also exposed to the API, `SCMTool` implementations, and extensions. It's all stored in namespaces within a `diffx` key in the `extra_data` fields in `DiffSet` (for the main DiffX section), `DiffCommit` (for change sections), and `FileDiff` (for file sections). Along with this, a diff's encoding is extracted and stored in a top-level `encoding` key in `FileDiff.extra_data` (which is an existing established key that aids in diff processing).