Add support for using TLA to configure libtap/settings

[coreyfarrell]

Top level await alters load ordering. For example if you have ./tap.js:

import './libtap-setup.js'
import t from 'libtap'

./libtap-setup.js:

import settings from 'libtap/settings'
import findUp from 'find-up'

const packageJSON = await findUp('package.json')
// read package.json and use it to alter settings

In this case tap.js was written to run the libtap-setup.js steps to completion before libtap is imported. Unfortunately having top level await in libtap-setup.js or anything it imports changes the order. libtap starts loading as soon as the first top level await is hit.

With this patch you would make two small changes. You would change import t from 'libtap' to import t from 'libtap/tla', then you would add a call to settings.markAsReady() after the settings changes are complete.

I'm not settled on this approach though it's the best I have right now.