Extend go-ts-mode with command to add docstring to function

Extend go-ts-mode with command to add docstring to function

go-ts-mode is extended with command go-ts-mode-docstring which adds
docstring comment to the defun at point. If a comment already exists,
the point is instead moved to the top-most comment line. The command
is bound to "C-c C-d".

* lisp/progmodes/go-ts-mode.el (go-ts-mode): Extend docstring.
(go-ts-mode-docstring): New function.
(go-ts-mode--comment-on-previous-line-p): New function.
(go-ts-mode-map): New map variable.
* etc/NEWS: Mention the change.

(bug#60805)