Support DAP's terminate request when ending a debug session

[weinand]

To fix issue microsoft/vscode-node-debug#1 we will add this feature to VS Code:

• if a debug adapter supports "terminate", VS Code's red terminate button will first send a "terminate" request to the DA (which some DAs might implement by sending a SIGINT signal to the debuggee). VS Code remembers in its debug session state that a "terminate" request has been issued.
• If the debuggee has no SIGINT handler, the debuggee will terminate and end the debug session. This is almost identical to the current behaviour.
• if the debuggee has a SIGINT handler but doesn't terminate properly, the debug session will continue. Pressing the red terminate button again will send a "disconnect" request. This will kill the debuggee forcefully (like the current behaviour).

@isidorn I've added a terminate to VA Code's DAP.

[isidorn]

I have pushed the inital changes needed for this.
We still only have one action "Debug: Stop" which when invoked only once sends the terminate request (if the capability is supported). Otherwise if the capability is not supported, or if triggered for the second time sends a "disconnect".

Things to keep an eye open for: sessions staying around when debugging is over. Also extensinos host debugging, restarting and reloading. I tested this out but we should test more.

[weinand]

@isidorn this request should not be used for attach debug configs. I've created #54420 for this.