This pull request includes several changes to the src/test-utils.ts file to improve the handling of the React act function and IntersectionObserver mocking. The key changes involve refactoring the code to ensure compatibility with different environments and simplifying the logic.

Refactoring for compatibility and simplification:

• Removed the global declarations for IS_REACT_ACT_ENVIRONMENT and jest in src/test-utils.ts, so we don't clash with user type definitions.
• Updated the condition to check for window before setting up IntersectionObserver mocking in src/test-utils.ts.
• Replaced the getIsReactActEnvironment function with getActFn to encapsulate the logic for determining the appropriate act function in src/test-utils.ts.
• Modified the triggerIntersection function to use the new getActFn method for invoking the act function in src/test-utils.ts.

Fixes #710 and #711