descr-text: add `describe-char-eldoc' describing character at point
* lisp/descr-text.el (describe-char-eldoc): New function returning
basic Unicode codepoint information (e.g. name) about character
at point. It is meant to be used as a default value of the
`eldoc-documentation-function' variable.
(describe-char-eldoc--format, describe-char-eldoc--truncate):
New helper functions for `describe-char-eldoc' function.
* tests/automated/descr-text-test.el: New file with tests for
`describe-char-eldoc--truncate', `describe-char-eldoc--format',
and `describe-char-eldoc'.