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/cmdexpand.c

@@ -2141,6 +2141,7 @@ static const char *set_context_by_cmdname(const char *cmd, cmdidx_T cmdidx, expa
     return set_context_in_scriptnames_cmd(xp, arg);
 
   case CMD_lua:
+  case CMD_equal:
     xp->xp_context = EXPAND_LUA;
     break;