Make js-beginning-of-defun return non-nil on success

Make js-beginning-of-defun return non-nil on success

The docstring of 'beginning-of-defun-function' says that the
function shall return non-nil when it found the beginning
of a defun.  This is specially important because the calling
code decides when to move point depending on the return value.
* lisp/progmodes/js.el (js-beginning-of-defun)
(js--beginning-of-defun-flat): Return non-nil when the beginning
of a defun is found.  (Bug#64283)

* test/lisp/progmodes/js-tests.el (js-mode-end-of-defun): Add a unit
test.