Avoid recomputing the whole table in 'vtable--recompute-numerical'

Avoid recomputing the whole table in 'vtable--recompute-numerical'

Each element of LINE being tested is a list, the first element of
which is the value actually being represented in the table.
Previously, the 'numberp' test would always fail, because it was
being compared with the list rather than the intended value in it;
that could cause the whole table to be recomputed, sometimes
unnecessarily.
* lisp/emacs-lisp/vtable.el (vtable--recompute-numerical): Test the
car of ELEM, not ELEM itself, which is a list.  (Bug#69927)

(cherry picked from commit b3f04eb68499f285e05b5b74e9cbd67f3140fb3c)