Automatically include sourcemap-register in source map builds

[guybedford]

This implements #180 including the sourcemap-register.js file as an asset of any ncc build -s call (it's 122KB). The build file is then updated to require this file first to provide source map support in Node.js for any execution errors.

[codecov-io]

Codecov Report

❗ No coverage uploaded for pull request base (master@bfe57db). Click here to learn what that means.
The diff coverage is 100%.

@@            Coverage Diff            @@
##             master     #270   +/-   ##
=========================================
  Coverage          ?   74.35%           
=========================================
  Files             ?       17           
  Lines             ?      889           
  Branches          ?        0           
=========================================
  Hits              ?      661           
  Misses            ?      228           
  Partials          ?        0

Impacted Files	Coverage Δ
src/cli.js	59.61% <ø> (ø)
src/index.js	69.86% <100%> (ø)

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