Add realpath library function

[joshblum]

No description provided.

[szwacz]

Thanks for the PR. I have problem finding windows dev machine to tinker with failed tests ;)
Will come back to it.

[joshblum]

Took a guess at what the windows problem could be but also don't have a windows dev machine setup. Could you try running CI again on the change? Thanks!

[codecov]

Codecov Report

Attention: Patch coverage is 97.45763% with 3 lines in your changes missing coverage. Please review.

Project coverage is 96.94%. Comparing base (79f53db) to head (f211f50).

Files	Patch %	Lines
lib/realpath.js	90.47%	2 Missing ⚠️
spec/realpath.spec.ts	98.86%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #121      +/-   ##
==========================================
+ Coverage   96.92%   96.94%   +0.01%     
==========================================
  Files          52       54       +2     
  Lines        4624     4740     +116     
  Branches       25       25              
==========================================
+ Hits         4482     4595     +113     
- Misses        123      126       +3     
  Partials       19       19              

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