fix: use webpack.sources when it's available

[timneutkens]

This PR contains a:

• bugfix
• new feature
• code refactor
• test update
• typo fix
• metadata update

Motivation / Use-Case

https://github.com/webpack/webpack/releases/tag/v5.0.0-beta.27

We ran into this issue with the latest serializer so @sokra exposed the webpack-sources classes on the sources property in webpack itself

Breaking Changes

Not breaking

Additional Info

[alexander-akait]

Tests are broken

[timneutkens]

Tests are broken

Seems like this is caused by a newer version of webpack being used and not really by this PR.

[codecov]

Codecov Report

Merging #301 into master will increase coverage by 0.01%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master     #301      +/-   ##
==========================================
+ Coverage   94.53%   94.55%   +0.01%     
==========================================
  Files           5        5              
  Lines         366      367       +1     
  Branches      143      144       +1     
==========================================
+ Hits          346      347       +1     
  Misses         19       19              
  Partials        1        1              

Impacted Files	Coverage Δ
src/index.js	96.06% <100.00%> (+0.01%)	⬆️

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[alexander-akait]

Thanks!