The docstring inserted with go-ts-mode's C-c C-d was incorrectly
prefixed with the receiver "(myStruct).":

    // (myStruct).act
    func (m *myStruct) act () {...}

The above docstring is not correct because the receiver "myStruct"
should not be in the docstring.

The issue is caused by imenu and go-ts-mode--defun-name using the same
code to determine the defun name. Instead, they should produce
different results - imenu should show the "myStruct" prefix, but the
docstring should not.

This commit fixes the incorrect behavior by introducing an optional
SKIP-PREFIX parameter to (go-ts-mode--defun-name). Pressing C-c C-d
now inserts just the method name:

    // act
    func (m *myStruct) act () {...}

* lisp/progmodes/go-ts-mode.el (go-ts-mode--defun-name): New optional
parameter SKIP-PREFIX. (go-ts-mode-docstring):
Call (go-ts-mode--defun-name t) instead of (treesit-defun-name).