Support numeric indexing in let-alist

Support numeric indexing in let-alist

let-alist is very useful.  But sometimes an alist contains a list in
the middle, which contains yet more alists.  Previously, this was
somewhat painful to deal with, and required something like:

(let-alist alist
  (let-alist (nth 0 .a)
    (let-alist (nth 3 .b)
       .c)))

Now, the following works:

(let-alist alist
  .a.0.b.3.c)

* lisp/emacs-lisp/let-alist.el (let-alist--access-sexp): Properly
parse numbers to handle lists.  (Bug#66509)
(let-alist--list-to-sexp): Use nth to handle numbers.
(let-alist): Update docs.

(cherry picked from commit ae4171efdc60dfa53aa404e5926f5165dbf98d59)