feat: upgrade to grumbler scripts 8

[dtjones404]

DTPPSDK-949

• Migrated to grumbler scripts v8.
• Added flow-typed, jest, and cross-env as explicit dev-dependencies.
• Added declaration for __DEBUG__ to resolve flow error

[codecov-commenter]

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Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 69.34%. Comparing base (288257d) to head (e96d7db).
Report is 36 commits behind head on main.

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Additional details and impacted files

@@           Coverage Diff           @@
##             main      #61   +/-   ##
=======================================
  Coverage   69.34%   69.34%           
=======================================
  Files           5        5           
  Lines         137      137           
=======================================
  Hits           95       95           
  Misses         42       42           

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