fix: stop caching existence of window

[meeber]

Caching the existence of window introduces problems in quasi-browser
environments where it's not uncommon to unset window before some tests.

Fixes #133

[codecov]

Codecov Report

Merging #134 into master will decrease coverage by 2.88%.
The diff coverage is 90%.

@@            Coverage Diff             @@
##           master     #134      +/-   ##
==========================================
- Coverage   85.71%   82.82%   -2.89%     
==========================================
  Files           1        1              
  Lines         105       99       -6     
==========================================
- Hits           90       82       -8     
- Misses         15       17       +2

Impacted Files	Coverage Δ
index.js	82.82% <90%> (-2.89%)	⬇️

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[keithamus]

Thanks for going through the motions with this @meeber. The whole window thing is a PITA, so thanks for perserveering!

[meeber]

@keithamus Happy to do it. And thanks to you and @astorije for helping out with all of these PR reviews.

[astorije]

No worries! I'm not helping much, you're doing the real work :)