Add support for running concurrent debug sessions

[mfussenegger]

Still needs some work

New API:

• dap.sessions() to return active debug sessions
• dap.ui.widgets.sessions to show active debug sessions

Step functions will change the focus automatically if the currently
focused session is not stopped.
This should make common scenarios like debugging client + server where
you step from making requests on the client to receiving request on the
server convenient.

Note that this is unrelated to startDebugging support. The PR here is
about concurrent top-level sessions. startDebugging support will
introduce hierarchical sessions. (Probably including something like
children in the Session object)

Open questions

• If there is an active session, should run_last / run default to restart (terminate, run new), or should they default to run an additional session? Will restart if config name matches, otherwise start additional session

recording.mp4

[mfussenegger]

@rcarriga heads up so you can check nvim-dap-ui with these changes as I'm probably going to merge this soonish. Overall for most users nothing should really change about their existing workflows. But you may want to create a sessions widget and one important difference is that if users do spawn multiple sessions, there can still be active sessions after receiving a terminated event (so closing the dapui on it would be wrong, one would have to check via dap.sessions() if it was the last)

[rcarriga]

Thanks for tagging! So there's no breaking change here, just as you say it'd be useful for users to be able to choose a session?

[mfussenegger]

just as you say it'd be useful for users to be able to choose a session?

Yes and that in general there could now be more than one active session. That affects listener triggers and so on