Fix: worker tasks parallel execution

[Maksimka101]

Previously on native platforms CombineWorker didn't payed atention to the tasksPerIsolate parameter and executed them sequentially

PR Checklist

• Code is 100% covered.
• I updated/added relevant documentation.
• All existing and new tests are passing.
• Changelog is updated.

• Version is pumped

[codecov-commenter]

Codecov Report

Merging #15 (8afaf1e) into master (0d4e335) will not change coverage.
The diff coverage is 100.00%.

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@@            Coverage Diff            @@
##            master       #15   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files           30        30           
  Lines          383       383           
=========================================
  Hits           383       383           

Impacted Files	Coverage Δ
lib/src/combine_worker/combine_task_executor.dart	100.00% <100.00%> (ø)