bug#64283: 29.0.91; js-mode's mark-defun does not work correctly when functions have a comment on top

[Yuan Fu]

> 
> What I see is that, after 4489450f37deafb013b1f0fc00c89f0973fda14a,
> defun movement may be subtly broken if beginning-of-defun-function does
> not return non-nil when it found the beginning of a defun.  One of the
> affected modes is js-mode, but who knows if there are more out there.
> 
> We could either revert 4489450f37deafb013b1f0fc00c89f0973fda14a, because
> of the incompatibilities it may cause (Yuan, what is the bug it tries to
> fix?), or maybe adjust js-mode so that it follows the documentation of
> beginning-of-defun-function and returns non-nil when it found the
> beginning of a defun.  I've attached a patch that follows this second
> approach, with some unit tests.  It fixes the bug on my side.
> 
> <0001-Make-js-beginning-of-defun-return-non-nil-on-success.patch>

The original problem that I tried to solve is that sometimes 
end-of-defun-function was called when point isn’t at the beginning of a defun, 
contrary to what the documentation claims. 

I first find out about it when writing defun movement functions for 
tree-sitter, but if you revert the commit now tree-sitter defun functions 
wouldn’t break: they have change quite a bit since then and 
treesit-end-of-defun don’t need to be called at the beginning of the defun 
anymore.

Yuan