Deprecate isTypeSupported in favor of Media Capabilities

[marcoscaceres]

It may not be possible to deprecate the isTypeSupported() API, but maybe we can encourage user agents to display a warning and also discourage further specify these capabilities for media elements and instead use w3c.github.io/media-capabilities/.

See also:
whatwg/html#10460

[joeyparrish]

Media applications have to support devices with non-updating web frameworks that are as many as 5 years old. So applications won't be able to completely give up on MediaSource.isTypeSupported for quite some time.

If there's a warning, it would only show up on newer devices and evergreen browsers, though, so that should be fine and probably won't cause much confusion.