Support stopping a single goroutine

[dlsniper]

Once https://github.com/golang/go/issues/31132 is implemented, Delve should also implement support for it.

[mangalaman93]

It'd be pretty useful to only execute one goroutine when next or step is executed. Currently, all the goroutine make progress, which makes it harder to debug a particular scenario when multiple goroutines are running. I am wondering though whether it is possible to achieve this without the support from runtime as mentioned in the issues https://github.com/golang/go/issues/25578 https://github.com/golang/go/issues/31132

[leos-code]

Hopefully, this feature will be supported