Flymake: new flymake-diagnostic-format-alist (bug#77439, bug#77480)

Flymake: new flymake-diagnostic-format-alist (bug#77439, bug#77480)

* lisp/progmodes/flymake.el (flymake-diagnostic-text): Overhaul.
(flymake-diagnostic-format-alist): New defcustom.
(flymake--diagnostic-format): New util.
(flymake--eol-overlay-summary)
(flymake--highlight-line)
(flymake-eldoc-function): Use flymake--diagnostic-format.
(flymake--tabulated-diagnostic-origin): New helper.
(flymake--tabulated-entries-1): Use flymake--tabulated-diagnostic-origin.
(flymake--diagnostics-base-tabulated-list-format): "Backend" -> "Origin"

* doc/misc/flymake.texi (Customizable variable): Mention new variable.

(cherry picked from commit ed1311a62a4e02817e548b0873ab21114047c076)