Use Babel directly

[novemberborn]

See #1789 for context.

Simplify test and helper file precompilation. Remove parallel Babel options resolution, at the cost of false cache positives. Fixes #1732. Fixes #1667.

• Currently the @ava/stage-4 preset cannot be disabled. See Preserve disabled plugins / presets in @babel/core loadPartialConfig() babel/babel#7920.
• I couldn't make the {modules: false} option work, see @babel/core's loadOptions() drop presets that have options babel/babel#7923
• I may be resolving the options incorrectly too, see Make @babel/core loadPartialConfig() resolve babelrc and config babel/babel#7919
• Still need to tackle Handle compilation of files ignored by Babel #1667
• Our plugin / preset dependencies need to be updated to the latest Babel beta
• I need to file a follow-up issue for testing with babel.config.js files Document usage of babel.config.js files #1816
• I need to file a follow-up issue for cache improvements Restore Babel cache invalidation #1817

[billyjanitsch]

This is awesome!

[novemberborn]

CI willing this is ready. I'm facing a busy week but hopefully I can land this next weekend or shortly after.

@billyjanitsch could you give this PR a try?