feat: use global speechSynthesisPolyfill and SpeechSynthesisUtterancePolyfill

Rename speechSynthesis into speechSynthesisPolyfill
and SpeechSynthesisUtterance into SpeechSynthesisUtterancePolyfill

Breaking change:
now is required to use speechSynthesisPolyfill and  SpeechSynthesisUtterancePolyfill

Closes #2
chore(demo): update to new polyfill names
docs(readme): update docs to new polyfill names [ci skip]
chore: v0.3.0