feat(lua): allow :=expr as a shorter version of :lua =expr

existing behavior of
  :=
and
  :[range]=

are unchanged. `|` is still allowed with this usage.

However,
   :=p
and similar are changed in a way which could be construed as a breaking
change. Allowing |ex-flags| for := in the first place was a mistake as
any form of := DOES NOT MOVE THE CURSOR. So it would print one line number
and then print a completely different line contents after that.

src/nvim/ex_cmds.lua

@@ -3319,7 +3319,7 @@ module.cmds = {
   {
     command='=',
     enum='CMD_equal',
-    flags=bit.bor(RANGE, TRLBAR, DFLALL, FLAGS, CMDWIN, LOCK_OK),
+    flags=bit.bor(RANGE, EXTRA, DFLALL, ARGOPT, CMDWIN, LOCK_OK),
     addr_type='ADDR_LINES',
     func='ex_equal',
   },