chore: update code coverage

[doctyper]

• merge multiple code coverage reports
• add Firefox to list of browsers to cover
• add test for requestIdleCallback

[codecov]

Codecov Report

Merging #254 into master will increase coverage by 0.36%.
The diff coverage is n/a.

@@            Coverage Diff            @@
##           master    #254      +/-   ##
=========================================
+ Coverage   98.54%   98.9%   +0.36%     
=========================================
  Files           3       3              
  Lines         275     275              
=========================================
+ Hits          271     272       +1     
+ Misses          4       3       -1

Impacted Files	Coverage Δ
src/HelmetUtils.js	98.57% <0%> (+0.47%)	⬆️

---

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[codecov]

Codecov Report

Merging #254 into master will increase coverage by 0.36%.
The diff coverage is n/a.

@@            Coverage Diff            @@
##           master    #254      +/-   ##
=========================================
+ Coverage   98.54%   98.9%   +0.36%     
=========================================
  Files           3       3              
  Lines         275     275              
=========================================
+ Hits          271     272       +1     
+ Misses          4       3       -1

Impacted Files	Coverage Δ
src/HelmetUtils.js	98.57% <0%> (+0.47%)	⬆️

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update 9afa44b...950b006. Read the comment docs.