refactor: es modules in client

[alexander-akait]

• This is a bugfix
• This is a feature
• This is a code refactor
• This is a test update
• This is a docs update
• This is a metadata update

For Bugs and Features; did you add new tests?

existing tests

Motivation / Use-Case

use ECMA modules there it possible

Breaking Changes

No

Additional Info

No

[codecov]

Codecov Report

Merging #3550 (800549f) into master (0c07404) will increase coverage by 0.49%.
The diff coverage is 50.00%.

@@            Coverage Diff             @@
##           master    #3550      +/-   ##
==========================================
+ Coverage   93.53%   94.02%   +0.49%     
==========================================
  Files          18       15       -3     
  Lines        1284     1256      -28     
  Branches      431      431              
==========================================
- Hits         1201     1181      -20     
+ Misses         77       69       -8     
  Partials        6        6              

Impacted Files	Coverage Δ
client-src/clients/SockJSClient.js	100.00% <ø> (ø)
client-src/clients/WebSocketClient.js	100.00% <ø> (ø)
client-src/utils/createSocketURL.js	100.00% <ø> (ø)
client-src/utils/getCurrentScriptSource.js	100.00% <ø> (ø)
client-src/utils/parseURL.js	88.23% <ø> (-1.77%)	⬇️
client-src/utils/reloadApp.js	82.60% <ø> (-2.01%)	⬇️
client-src/utils/sendMessage.js	83.33% <ø> (-2.39%)	⬇️
lib/utils/DevServerPlugin.js	100.00% <ø> (+2.17%)	⬆️
client-src/index.js	85.56% <40.00%> (-1.23%)	⬇️
client-src/utils/log.js	100.00% <100.00%> (ø)
... and 3 more

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[wojtekmaj]

Would be wise to link to the bug for future reference