Make VS-15 and VS-16 compositions work correctly

Make VS-15 and VS-16 compositions work correctly

There is a conflict between forward matching and backward matching
composition rules involving the same codepoint, which can cause the
backward matching ones not to be invoked.  Ensure that VS-15 (U+FE0E)
and VS-16 (U+FE0F) are composed by forward matching rules instead in
order to avoid this issue.

* admin/unidata/emoji-zwj.awk: Add rules for CHAR+VS-15 and CHAR+VS-16.
* lisp/composite.el: remove backward matching rule for VS-15.  (Bug#63731)