Add transaction isolation support

[ayroblu]

Attempt to solve #581
Probably needs some extra tests, not sure where the best place to put them is.
Also probably needs to implement for all drivers rather than just half of them

Syntax is knex.transaction(...).setIsolationLevel('string')

[ayroblu]

Urg I don't get why it's still failing, it passes fine locally :/

And I get unrelated failures when I run the whole thing

[kibertoad]

@clemens-smartparking Unfortunately, that is not that straightforward. First of all, Objection TS types need to be adjusted, as seen in Vincit/objection.js#1992. Second, looks like Objection is importing some of knex internal files directly, so that will need to be adjusted as well. I'll spend some time this week to fix incompatibilities on Objection side, hopefully some kind of next release can be done for Objection as well.