test: loader, update jest

[morganney]

• Add test for the loader, update Jest.

[codecov]

Codecov Report

Merging #6 (8e7e5e1) into master (32e93c2) will increase coverage by 32.14%.
The diff coverage is n/a.

@@             Coverage Diff              @@
##           master        #6       +/-   ##
============================================
+ Coverage   67.85%   100.00%   +32.14%     
============================================
  Files           2         2               
  Lines          28        28               
  Branches       10        10               
============================================
+ Hits           19        28        +9     
+ Misses          8         0        -8     
+ Partials        1         0        -1     

Impacted Files	Coverage Δ
src/loader.js	100.00% <0.00%> (+100.00%)	⬆️

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