feat(build): Migrate to mobx version 5

[gristow]

• Upgrade MobX peer dependency to >= 5.15.4
• Change tsconfig target to es2015 to match MobX 5 target
• Upgrade Webpack & loaders to v. 4.43
• Update parcel and revises /example/.babelrc to match the use of Babel 7 in Parcel >= 1.10.0
• Update jest

[codecov-io]

Codecov Report

:exclamation: No coverage uploaded for pull request base (master@93b768f). Click here to learn what that means. The diff coverage is n/a.

@@            Coverage Diff            @@
##             master      #23   +/-   ##
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  Coverage          ?   93.75%           
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  Files             ?        1           
  Lines             ?       16           
  Branches          ?        3           
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  Hits              ?       15           
  Misses            ?        0           
  Partials          ?        1           

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