rationale behind the addition of this file is it's meant to replace
TestCasePackager.ps1 outright and will be runnable on all major
operating systems.  There's still a bit of cleanup work needed as
the Python port was hardly optimized for the language.

A few side effects of this change:
- it now takes under ten seconds to generate the test262 website (PS
  version was taking over a minute)
- generated *.json files are ~5% smaller in size (PS was forcing
  '\r\n' and the Python version preserves the tests' newlines whether
  they're '\r\n' or '\n')