fix: sourcemap generating when previous loader pass sourcemap as string

[alexander-akait]

This PR contains a:

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

Motivation / Use-Case

Previously we stringify string, it is invalid. No test, because we already have test on this

Breaking Changes

No

Additional Info

No

[codecov]

Codecov Report

Merging #905 into master will not change coverage.
The diff coverage is 100%.

@@           Coverage Diff           @@
##           master     #905   +/-   ##
=======================================
  Coverage   99.24%   99.24%           
=======================================
  Files          10       10           
  Lines         396      396           
  Branches      118      118           
=======================================
  Hits          393      393           
  Misses          3        3

Impacted Files	Coverage Δ
src/index.js	100% <100%> (ø)	⬆️

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