Don't compile Hermes-supported features with Babel

[gaearon]

This simplifies the generated code to be closer to the code we actually write.

Concretely:

• Hermes already supports a bunch of features that we're currently compiling with Babel. Pass an option to target the subset needed by Hermes specifically, and let Hermes handle the rest.
• Although Hermes supports object spread, the Expo preset adds it back. Explicitly patch the Expo preset to disable it.

We will be able to remove both of these overrides when we upgrade Expo:

• The transform profile is being auto-detected now.
• The object spread plugin is conditional now.

However, I don't know when we'll get to updating, so why not fix this now.

Test plan

Walked around the app. App seems to work. Equivalent changes being in newer Expo releases give some confidence.

Bundle output changes

Glance over the bundle diff.

Function names are now implied (since Hermes can do this):

Native support for ?? and ?. and friends:

No runtime code needed for rest/spread:

Also, we finally get rid of this _objectSpread helper:

Fun fact: this _objectSpread helper was duplicated in our bundle almost three thousand times 500 times (with 3000 call sites).

Total minified PROD bundle went down from 8.9 MB to 8.6 MB.

Spreads in particular are very common so having them go through fewer indirections is nice.

[pfrazee]

Ahhh very nice