Allow debuggers to support StepBack without ReverseContinue

[DanTup]

The "ReverseContinue" action is controlled by the same supportsStepBack flag that "StepBack" is. This means for any debugger than can support step back, but not reverse continue, there will be an extra button on the debug toolbar that won't function.

[vscodebot]

(Experimental duplicate detection)
Thanks for submitting this issue. Please also check if it is already covered by an existing one, like:

• Step back and reverse continue have wrong icons (#85111)

[weinand]

Adding a new capability or changing the existing ones in a backward compatible way would be a mess.
No plans to do this.

[DanTup]

I forgot to post back here - I was actually using the wrong thing here anyway (I was implementing RestartFrame). It's done correctly now so I'm not using this now :-)