feat(#2544): add API for querying win ID, api.tree.winid()

[j-hui]

Closes #2544

[alex-courtis]

Looking great! One failure with 0 tabpage.

Testing:

no tabpage:

• startup - nil
• tree open - valid
• tree closed - nil
• create tab 2 - nil
• tab 2 tree open - valid
• tab 1 tree open - valid
• tab 2 tree closed - nil

tabpage 1 provided:

• startup - nil
• tree open - valid
• tree closed - nil
• create tab 2 - nil
• tab 2 tree open - nil
• tab 1 tree open from tab 2 - valid
• tab 2 tree closed - valid
• tab 2 and tab 1 tree closed - nil

tabpage 0 provided:

• always nil

[j-hui]

Thanks for your quick and detailed feedback! I just pushed a change to address that issue you found when tabpage is 0.

[alex-courtis]

Works nicely for 0 and nil; many thanks for your contribution!