Add seq-find

Add seq-find

This function is similar to `seq-some' but returns the found element.
In the cases where nil can be the found element, a sentinel optional
argument can be provided to avoid ambiguities.

* lisp/emacs-lisp/seq.el (seq-find): New function.
* test/automated/seq-tests.el (test-seq-find): Add tests for `seq-find'.
* doc/lispref/sequences.texi (Sequence Functions): Add documentation for
seq-find.