This PR adds a new `@container-size` utility instead of
`@container-[size]`.
The main reason we didn't do this before is because we only have
container width related container queries, and not block based ones so
we never needed `size` and `inline-size` was enough.
However, `@container-size` is still useful if you are using container
query related units such as `cqb` which are using the block size of the
container not the inline size.
I also added a little helper such that `@container-size` is only
available in `insiders` and `4.2.0` (and later) so `4.1.x` releases
won't have this utility yet. This will require some CHANGELOG changes
such that we don't include this when releasing the next minor release.
b7c7e48c5d