Add container property to source Element to keep up with CSS @container-queries

[MajPay]

Currently - the media property of source elements gives you the ability to control which source the browser should pick from a larger set of possible sources. This way - as a web-designer - you try to achieve responsive image Implementations.

CSS @media-queries were nice when they were first introduced, but they soon turned out to not be sufficient for real-world web apps. So the logical successor will be @container-queries. To keep up with that development, the source Element should also implement a container property in the future.