Add `next-error' support for flymake diagnostics buffers

Add `next-error' support for flymake diagnostics buffers

This adds `next-error' support for flymake diagnostics buffers.
Buffers created with `flymake-show-buffer-diagnostics' and
`flymake-show-project-diagnostics' are now next-error enabled,
and `next-error' and `previous-error' will navigate through
their listed diagnostics.
* lisp/progmodes/flymake.el (flymake-current-diagnostic-line)
(flymake--diagnostics-next-error): Add.
(flymake-show-diagnostic, flymake-show-buffer-diagnostics)
(flymake-show-project-diagnostics): Set next-error-last-buffer.
(flymake--tabulated-setup): Set next-error-function.
(Bug#77809)

Copyright-paperwork-exempt: yes
(cherry picked from commit ee8b4eaca12af0ef6bf0d37780efaa558c25d45e)