fix: serializing big strings

[alexander-akait]

This PR contains a:

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

Motivation / Use-Case

fixes #643

Breaking Changes

No

Additional Info

No

[sokra]

Need to be converted back to string here:

https://github.com/webpack-contrib/mini-css-extract-plugin/blob/69aa92bc9214aedd942f5a5efab5454e9ae4f329/src/index.js#L662

[codecov]

Codecov Report

Merging #647 into master will not change coverage.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master     #647   +/-   ##
=======================================
  Coverage   89.55%   89.55%           
=======================================
  Files          11       11           
  Lines         651      651           
  Branches      181      181           
=======================================
  Hits          583      583           
  Misses         65       65           
  Partials        3        3           

Impacted Files	Coverage Δ
src/loader.js	92.03% <ø> (ø)
src/index.js	91.35% <100.00%> (ø)

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