Consider stabilizing `SendChannel.isClosedForSend`

[JakeWharton]

It was added in 0.7(!!) and made experimental in 1.0 about two years later. That was now 4 years ago. Time to stabilize?

[qwwdfsad]

Makes sense.

The biggest blocker was its non-linearizability for buffered channel, but its unlikely to bother anyone and can be changed even in minor release

[qwwdfsad]

We are going to promote it to delicate API along with isClosedForReceive