(syntax-wholeline-max): New var

(syntax-wholeline-max): New var

Try and reduce the pain caused by font-lock and syntax-propertize's
wholeline-based operation in buffers made up of a few very long lines
(bug#45898).

* lisp/emacs-lisp/syntax.el (syntax-wholeline-max): New var.
(syntax--lbp): New function.
(syntax-propertize-wholelines): Use it.

* lisp/jit-lock.el (jit-lock--antiblink-post-command): Use `syntax--lbp`.

* lisp/font-lock.el (font-lock-extend-region-wholelines): Rewrite,
using `syntax-propertize-wholelines`.