Implement the plugin as a babel macro

[emmatown]

What:

Allow people to use the plugin as a babel macro.

import styled from 'emotion/react/macro'

import { css } from 'emotion/macro'

Why:

#132

How:

Abstract stuff in babel.js

Checklist:

• Documentation
• Tests
• Code complete

I based this off #130 so I could make the react styled import emotion/react/macro (we need to have separate macros for each runtime version of styled since we have to add imports to the runtime, I'm gonna abstract this away so it won't have a big maintenance cost)

This is very work in progress and it will have to change a bunch.

The only thing that we can't do with a babel macro is the css prop. (I think)

• styled tagged template literal
• styled object
• css
• keyframes
• injectGlobal
• fontFace
• commonjs requires babel-macros only supports importing defaults with commonjs so just supporting this for styled doesn't seem worth it imo. Throw an error if the macro is imported with commonjs
• Vue
• import paths; emotion/macro, emotion/react/macro, emotion/vue/macro
• use ES imports to import runtime
• export things in index that don't need transforming from macro

Closes #132
Closes #112

Just a note: #130 should be merged before this

[codecov-io]

Codecov Report

Merging #142 into master will increase coverage by 1.07%.
The diff coverage is 98.13%.

@@            Coverage Diff             @@
##           master     #142      +/-   ##
==========================================
+ Coverage   89.08%   90.15%   +1.07%     
==========================================
  Files          18       22       +4     
  Lines         632      701      +69     
  Branches      145      167      +22     
==========================================
+ Hits          563      632      +69     
  Misses         56       56              
  Partials       13       13

Impacted Files	Coverage Δ
src/vue/index.js	100% <ø> (ø)
src/macro-styled.js	100% <100%> (ø)
src/react/macro.js	100% <100%> (ø)
src/babel-utils.js	100% <100%> (ø)
src/macro.js	100% <100%> (ø)
src/babel.js	98.11% <96.7%> (-0.02%)	⬇️

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