Add action_timing to _trigger space

This is due to a suggestion from Konstantin Osipov:
"

1. a feature request to extend _trigger system space with trigger
   timing - this should take a bit longer, yet we need to do it in
   2.2.
   "

The context was a mention of a column in information_schema.triggers
named ACTION_TIMING, which can be 'BEFORE' or 'AFTER' or 'INSTEAD OF'.
So add a field in the _trigger space containing one of those
values, depending on the way that CREATE TRIGGER was done.

I hope this feature request isn't due to a misunderstanding.
I mentioned that information_schema.triggers.ACTION_TIMING
can be hard to find by looking at the _trigger space.
But it is only one of several bits of information.
Therefore I suggested I would like a list of tokens.
That is, if I could say:
gimme_tokens([[box.execute([[create trigger before /* before */ After/before/INSERT ON t
FOR EACH ROW BEGIN DELETE FROM before; end;]])
then I could find ACTION_TIMING (which = 'AFTER' in this example),
and EVENT_MANIPULATION (= 'INSERT'), and EVENT_OBJECT_TABLE (= 'T').
For simulating information_schema, action_timing is not good enough.

Start with the _trigger space and the CREATE TRIGGER handling described in the issue, then compare the available metadata with information_schema.triggers.ACTION_TIMING. Done means the _trigger space exposes whether each trigger was created as BEFORE, AFTER, or INSTEAD OF.