Fix for using SDK as ESM

[joawan]

Importing the SDK as real ESM does not work due to

1. Missing file extension
2. Importing json

[codecov]

Codecov Report

Merging #256 (7289e12) into master (6cc771b) will increase coverage by 0.00%.
The diff coverage is 100.00%.

❗ Current head 7289e12 differs from pull request most recent head f626fec. Consider uploading reports for the commit f626fec to get more accurate results

@@           Coverage Diff           @@
##           master     #256   +/-   ##
=======================================
  Coverage   99.78%   99.79%           
=======================================
  Files          13       13           
  Lines         476      478    +2     
  Branches       96       96           
=======================================
+ Hits          475      477    +2     
  Misses          1        1           

Files	Coverage Δ
src/RESTClient.js	100.00% <ø> (ø)
src/cache.js	100.00% <ø> (ø)
src/identity.js	99.52% <100.00%> (+<0.01%)	⬆️
src/monetization.js	100.00% <100.00%> (ø)
src/object.js	100.00% <ø> (ø)
src/payment.js	100.00% <ø> (ø)
src/popup.js	100.00% <ø> (ø)
src/spidTalk.js	100.00% <ø> (ø)
src/url.js	100.00% <ø> (ø)
src/validate.js	100.00% <ø> (ø)

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