Provide an API to check whether debug mode is enabled

[qwwdfsad]

We already have a notion of debug mode in our documentation, but we do not have an API to check whether the mode is enabled.

It can be useful in multiple scenarios:

• To write unit tests that work both with and without debug mode
  • We already do that in our MPP tests
• To verify and debug program behaviour (see #2550)
• To write your own extensions to coroutines that may exhibit an additional behaviour when debug mode is enabled (stacktrace for cancellation exception subclasses, context markers and so on)

[qwwdfsad]

Also, it's worth adding unwrap function and separate stacktrace recovery check.

We already bump into this in our own modules: https://github.com/Kotlin/kotlinx.coroutines/pull/2646/commits/89fd2c179efcd5a80d5dd2f12ada5a75734e9326