Add completion-tab-width to align prefix chars with names in read-char-by-name

Add completion-tab-width to align prefix chars with names in read-char-by-name

* lisp/international/mule-cmds.el (mule--ucs-names-affixation):
Replace mule--ucs-names-annotation to display chars in prefixes
that implements two FIXME items.
(read-char-by-name): Let-bind completion-tab-width to 4.
Use affixation-function instead of annotation-function.

* lisp/minibuffer.el (completion-tab-width): New variable.
(completion--insert-strings): Align colwidth to tab positions
when completion-tab-width is non-nil.

* lisp/simple.el (completion-setup-function): Set tab-width to
completion-tab-width when completion-tab-width is non-nil.

https://lists.gnu.org/archive/html/emacs-devel/2020-11/msg01263.html