chore: upgrade to babel v7

[nolanlawson]

What this PR does:

• updates babel and its plugins/presets to v7 or equivalent
• updates jest to latest
• makes the dist-modern build use @babel/env with a list of latest Chrome/Edge/Firefox/Safari versions, rather than explicitly specifying every transform
• makes the legacy dist and dist-es builds use @babel/env with its default settings (i.e. run all ES2015 transforms)
• makes @babel/runtime an explicit dependency rather than devDependency
• removes most of the src/polyfills

I figure we are better off just making @babel/runtime into an explicit dependency rather than trying to write our own. Based on my testing, it adds a minimal amount of overhead (~4kB), and it's unlikely that our own polyfills will beat the built-in Babel ones. Plus it's just a lot of extra code we have to maintain.

Also, we no longer have to worry about bugs like #393 where a core-js dependency accidentally sneaks into our build and we break anyone who doesn't have this dependency.

Future TODOs:

• remove _JSON, it shouldn't be needed anymore (refactor: remove _JSON #402)
• make the requestAnimationFrame polyfill only run in Node (it's not needed in the browser) (fix: remove rAF polyfill, add SSR test #401)