New jit-lock-antiblink-grace feature

* lisp/jit-lock.el (jit-lock-antiblink-grace): New defcustom.
(jit-lock--antiblink-line-beginning-position)
(jit-lock--antiblink-string-or-comment): New variables
(jit-lock--antiblink-post-command): New helper.
(jit-lock-mode): Tweak post-command-hook and
jit-lock-context-timer.

* etc/NEWS: Mention jit-lock-antiblink-grace
1 job for master in 90 minutes and 2 seconds (queued for 2 seconds)
Status Job ID Name Coverage
  Test
failed #4428
test-all

01:30:02

 
Name Stage Failure
failed
test-all Test The script exceeded the maximum execution time set for the job
  GEN      lisp/eshell/eshell-tests.log
ELC lisp/faces-tests.elc
GEN lisp/faces-tests.log
ELC lisp/ffap-tests.elc
GEN lisp/ffap-tests.log
ELC lisp/filenotify-tests.elc
GEN lisp/filenotify-tests.log
Pulling docker image gitlab/gitlab-runner-helper:x86_64-d0b76032 ...
ERROR: Job failed: execution took longer than 1h30m0s seconds