feat. Better typescript with babel 7. implements #884

[theKashey]

Update babel plugin, update test, and clarify readme.

[codecov-io]

Codecov Report

Merging #958 into master will increase coverage by 0.47%.
The diff coverage is 100%.

@@            Coverage Diff            @@
##           master    #958      +/-   ##
=========================================
+ Coverage   88.42%   88.9%   +0.47%     
=========================================
  Files          30      30              
  Lines         674     739      +65     
  Branches      156     173      +17     
=========================================
+ Hits          596     657      +61     
- Misses         64      68       +4     
  Partials       14      14

Impacted Files	Coverage Δ
src/babel.dev.js	94.64% <100%> (ø)
babel.js
src/proxy/inject.js	77.27% <0%> (+1.34%)	⬆️

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