feat(core): optimize UTF-8 encoding

[sranka]

This PR optimizes UTF-8 encoding in the browser by using TextEncoder API.

• CHANGELOG.md updated
• Rebased/mergeable
• A test has been added if appropriate
• yarn test completes successfully
• Commit messages are in semantic format

[codecov-io]

Codecov Report

Merging #272 into master will not change coverage.
The diff coverage is 100.00%.

@@            Coverage Diff            @@
##            master      #272   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files           28        28           
  Lines         1003      1006    +3     
  Branches       228       228           
=========================================
+ Hits          1003      1006    +3     

Impacted Files	Coverage Δ
packages/core/src/impl/browser/FetchTransport.ts	100.00% <100.00%> (ø)

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