feat: use simpler env vars for win32

[CanadaHonk]

Just use environment variables PROCESSOR_ARCHITEW6432/PROCESSOR_ARCHITECTURE on win32 instead of checking paths. Closes #26.

Bonus when running as x86: probably much faster and works with no IO access. Also ~10% smaller filesize.

[XhmikosR]

Agreed, this is a lot simpler and probably faster. @feross do you see something wrong with this approach?

[CanadaHonk]

Fixed formatting to be like the rest of the repo.

[CanadaHonk]

Updated to return 'arm64' on such systems now too.

[CanadaHonk]

Removed now unused imports in index.js.