Implement offsets for absolute line numbers

Implement offsets for absolute line numbers

* src/xdisp.c (syms_of_xdisp) <display-line-numbers-offset>:
New variable to add an offset to absolute line numbers.
(syms_of_xdisp) <display-line-numbers>: Mention it in docstring.
(maybe_produce_line_number): Use it.

* doc/emacs/display.texi (Display Custom): Document it.

* etc/NEWS (value): Announce it.

* lisp/frame.el: Add `display-line-numbers-offset' to list of
variables which should trigger redisplay of the current buffer.