Extract common code for adding text properties

* lisp/font-lock.el (font-lock--add-text-property):
New function.
(font-lock-prepend-text-property)
(font-lock-append-text-property): Use it.

(Bug#35476)
1 job for master in 51 minutes and 50 seconds (queued for 1 second)
Status Job ID Name Coverage
  Test
passed #1749
test-all

00:51:50