Switch from a monorepo to a single package

[Rowno]

This switches Evergreen from monorepo to a single package. Closes #117

TODO

Code:

• Add tree shaking support
• Add new release process
• Update README
• Migration guide
• Fix the docs site

Non-code:

• Delete git tags
• Test prerelease
• Publish package
• npm deprecation warnings
  https://docs.npmjs.com/cli/deprecate

Migration guide

To migrate to the new evergreen-ui package, you just need to install the new package (yarn add evergreen-ui). Combine all your Evergreen imports (the export names haven't changed), e.g:

import TextInput from 'evergreen-text-input'
import SelectMenu from 'evergreen-select-menu'

Becomes:

import { TextInput, SelectMenu } from 'evergreen-ui'

And then remove all the old evergreen-* packages.

To ease the migration, we've created a command line tool to automate the process of updating all your imports.

The only breaking API change is that evergreen-avatar's getInitials function is no longer exported.

[vadimdemedes]

This is awesome!

[stevenmiller888]

💥