feat: identify the polyfill usage

Add `isPolyfill` variables to the speechSynthesis objects that indicates the polyfill usage instead of native functionality.

Fixes #3
test: add tests for the polyfill usage

Add `isPolyfill` tests

#3
docs(readme): add isPolyfill identification [ci skip]

Add `isPolyfill` identification

#3
Merge pull request #4 from janantala/feat/identify

Identify the polyfill usage