Sphinx and docutils lack any ability to work with modern <img srcset>

attributes, making it difficult to work with responsive/high-DPI images.

We've historically provided a retina_images extension that made use

of a legacy JavaScript library for swapping images at runtime based on

data attributes, but we don't even ship this anymore on our sites.
This change introduces a new extension that gives us a nice way of using

modern srcsets.
It extends the existing image directive to take an optional

sources option. If provided, this will determine the images that

will be collected in the resulting documentation and provided as a

srcset=. This looks like:

.. image:: path/to/file.png
   :sources: 2x path/to/file@2x.png
             3x path/to/file@3x.png
             ...


The structure is a bit different from the native value for

<img srcset>. Descriptors (e.g., 2x) go first, commas aren't

required if splitting across multiple lines, and 1x is inferred from

the main image.
This is actually entirely optional. If any suitable files exist

alongside the main referenced file, they'll be found and used to build

up the srcset. So the above file@2x.png and file@3x.png don't

even need to be specified. They'll be directly scanned and added.
This makes this extension a drop-in option for codebases without needing

to rely on the custom option.
The reason we're modifying image to begin with, rather than adding a

new directive, is in part to faciliate a drop-in solution that can still

render images without the use of the extension (such as when viewed on

GitHub), but also because the image machinery in both docutils and

Sphinx is baked in and complicated. It's far more error-prone to

introduce something new that replicates all that logic than to

monkeypatch with an optional option that doesn't even need to be

specified for the common case.
This extension does not depend on anything else within this package, and

requires no configuration, so it can be used directly in any Sphinx

documentation packages.

Unit tests pass.
Manually tested with our Review Board documentation, both with manual

sources and source scanning, and saw that the resulting docs looked correct

on standard and high DPIs, with srcset propery filled in.