forward-sexp and backward-sexp are incoherent

[DamienCassou]

Given this content

values
    .filter((a) => a)
    .filter((a) => a);

in a js2-mode buffer, forward-sexp seems to work fine but backward-sexp goes back to the beginning of the first line when invoked with point at the end of second line.

More information at AmaiKinono/puni#9.

[dgutov]

Yes, there is certain imbalance in their working. It has been that way for basically ever, I think.

I'll look into this when I have the time. If someone would like to propose a patch, though -- welcome.