This change defines a TypedDict for diff chunks, and a NamedTuple

for diff lines, with definitions and documentation for each element.
Before, these were just an opaque dictionary, and a heterogenous list.

The diff lines were particularly annoying because they were variable

length (if there was no metadata, that element was just left off the

end), and types were annoying (for "real" line numbers, we'd have an

int if the line was present on that side of the diff, but was set to

an empty string if it wasn't). This change technically breaks

compatibility in those two cases. We now always include the metadata

field, set to None if there's no metadata for the line. In the case of

real line numbers which are not present on one side of the diff (for

example, the line number of an added line for the left-hand side of the

diff), we set the data for that to None.
The diff_lines template tag and other consumers now use names instead

of indexing magically into the structure, making it much clearer what's

going on.
This also updates the regions field to always be a list. In some cases

we had empty lists, and in others we had None.
This change only touches code related to chunks and lines. Other diff

viewer cleanup is coming in later changes.

Tested a ton of different diffs, both in the diff viewer as well as

  the text and markdown Review UIs. Verified that all types of edits

  (added/edited/deleted/moved) lines all worked as expected.
Ran unit tests.