Promote CopyableThrowable to stable

[qwwdfsad]

The main benefit of making it stable is to enable other libraries that depend on kotlinx.coroutines leverage stacktrace.
E.g. retrofit is not ready to do so unless CopyableThrowable is stable: square/retrofit#3474

We should evaluate potential enhancements and interactions with (also potential) global exception registry for exceptions beyond user's control and promote it to stable

[dmarin]

will this be stable on 1.5.0?

[qwwdfsad]

No such plans for 1.5.0 unfortunately

[almozavr]

Retrofit is the de factor standart library for http in android world, and due to this issue it's really hard to read API error reports without hacky solutions, they are almost useless, it really hurts 😢

[LouisCAD]

What's blocking this becoming stable? API design concerns? Multiplatform support? Something else?