chore: Mark package as side effects free

[just-boris]

Issue #, if available:

Description of changes:

By default, bundlers are conservative and consider Symbol.for(...) call as a side effect and leave it in the bundle even if unused:

https://github.com/cloudscape-design/component-toolkit/blob/d5f68a152bf6f5f4226bc983d2c7d341600f5e0e/src/internal/global-flags/index.ts#L4C39-L4C78

Let's help them with a declaration

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[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 99.22%. Comparing base (d5f68a1) to head (7e15ded).
Report is 1 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #108   +/-   ##
=======================================
  Coverage   99.22%   99.22%           
=======================================
  Files          27       27           
  Lines         644      644           
  Branches      172      172           
=======================================
  Hits          639      639           
  Misses          5        5           

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