Implement $app/env

[Rich-Harris]

This adds an $app/env module that exports browser and dev:

<script>
  import { browser, dev } from '$app/env';
</script>

<p>Hello from the {browser ? 'client' : 'server'} in {dev ? 'dev' : 'prod'} mode!</p>

Right now, browser = typeof window !== 'undefined' which is brittle (would fail if someone did global.window = 42). Ideally we'd use import.meta.env.SSR instead but there's a blocker (#194).

I'm anticipating that amp would go here too if we do #160.