Use loose mode for class properties transform

[Andarist]

The main concern of this PR is to use loose mode for class properties transform. I took an opportunity to make a small babel config cleanup - instead of copying it to rollup.config.js and risking both it & babelrc being out of sync I've merged them both and put everything babel-related into babelrc.js file

[codecov]

Codecov Report

Merging #366 into master will not change coverage.
The diff coverage is n/a.

@@          Coverage Diff          @@
##           master   #366   +/-   ##
=====================================
  Coverage     100%   100%           
=====================================
  Files           9      9           
  Lines         205    205           
  Branches       61     61           
=====================================
  Hits          205    205

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[erikras]

Published in v3.7.0.

[lock]

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