Feature support: yaml-pro-ts-down-level

[uqix]

Similar to yaml-pro-ts-up-level, add yaml-pro-ts-down-level so that we can jump to the first child of the current key; then with next/prev subtree, we can jump to anywhere which is so handy with repeat-map.

[zkry]

I like the idea. I'll work on getting this in 👍

[uqix]

Down on a key without children results in this error:

Debugger entered--Lisp error: (wrong-type-argument integer-or-marker-p nil)
  yaml-pro-ts-down-level()
  funcall-interactively(yaml-pro-ts-down-level)
  command-execute(yaml-pro-ts-down-level)

[zkry]

Thanks, should be good now in the latest master.