@type attribute on <source> elements

I'm starting a new thread about the @type attribute, as requested by Adrian
Roselli.

I believe it is critical that we REQUIRE browsers to SKIP source elements
which have an unrecognized (or unsupported) mime-type value in the @type
attribute.

Otherwise, we will not be able to introduce to formats and simplify
<picture> in the future.

@type should be an OPTIONAL attribute, not required, but if present,
browsers should handle it in a specific way. Widely supported formats like
jpeg, png, and gif do not need a type="" attribute, but webp and future
formats do.

This will allow us to introduce new image formats in a backwards-compatible
manner.

-Nathanael Jones