fix: types for the parallel option

[alexander-akait]

This PR contains a:

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

Motivation / Use-Case

fixes #447

Breaking Changes

No

Additional Info

No

[codecov]

Codecov Report

Merging #448 (deab997) into master (210a5a0) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master     #448   +/-   ##
=======================================
  Coverage   97.18%   97.18%           
=======================================
  Files           4        4           
  Lines         284      284           
  Branches      107      107           
=======================================
  Hits          276      276           
  Misses          8        8           

Impacted Files	Coverage Δ
src/index.js	96.88% <ø> (ø)

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