Add text-expander-committed event

[Cbodfield]

Adding a new event, text-expander-committed, that is dispatched after the underlying input's DOM value has been updated.

This is useful for allowing subscribing listeners to be able to handle when the DOM is updated. The particular use case this is solving is allowing us to sync up with React state when using text-expander.

cc @theinterned

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[broccolinisoup]

👋🏼 @keithamus This seems like a web component PR, would you mind reviewing it? Thank you!

[keithamus]

LGTM