fix: Use head instead of body to detect edge pseudo var bug

[aomarks]

If this script is invoked in the head, then the body won't exist yet, which causes the appendChild line to throw.

I confirmed that the bug is still detected in Edge 15.15063.

[codecov-io]

Codecov Report

Merging #4982 into master will increase coverage by 0.29%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master    #4982      +/-   ##
==========================================
+ Coverage   98.79%   99.08%   +0.29%     
==========================================
  Files         122      122              
  Lines        5817     5817              
  Branches      762      762              
==========================================
+ Hits         5747     5764      +17     
+ Misses         69       52      -17     
  Partials        1        1

Impacted Files	Coverage Δ
packages/mdc-ripple/util.ts	97.56% <100%> (+4.87%)	⬆️
packages/mdc-checkbox/component.ts	96.87% <0%> (+1.04%)	⬆️
packages/mdc-tab/component.ts	98.36% <0%> (+3.27%)	⬆️
packages/mdc-base/component.ts	100% <0%> (+3.57%)	⬆️
packages/mdc-ripple/component.ts	100% <0%> (+3.77%)	⬆️
packages/mdc-auto-init/index.ts	100% <0%> (+4.16%)	⬆️
packages/mdc-switch/component.ts	98.27% <0%> (+6.89%)	⬆️
packages/mdc-radio/component.ts	95.83% <0%> (+8.33%)	⬆️

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[aomarks]

@abhiomkar Should be mergeable now. Fixed the failing test (needed to add head to the mock window), and added a semantic prefix to the PR title.