Fixes and improvements to react-intersection-observer/test-utils

• always recreate the IntersectionObserver mock (c720914)
• improve test lib detection (1858e43)
• use isMockFunction to detect if the IntersectionObserver is being mocked (189133f)

• restore beforeEach in test utils (788dd47)

• ensure browser environment in test-utils (6e459f2)

• support React 19 (8bafe25)
• Drop support for React 15/16. The types are no longer compatible with later version of React, since they have dropped the deprecated types.

• align IntersectionObserver options with DOM API (abb4ee8)

• fix package exports and simplify build (#695) (c9cef8f) - All thanks go to @RobinTail

• upgrade to Vitest 2 (#690) (054adf4)
  • Upgrade to Vitest 2
  • Add basic browser mode test, that runs with an unmocked IntersectionObserver
  • Types cleanup in test-utils, getting rid of @types/jest (replaced with vitest)
  • Move test mocking setup from beforeEach to beforeAll

• remove script to publish dist dir (0980efa)

• test-utils - execute mockClear method only when it exists (712ae37)

• support old act from react-dom (0e95b5d)