ReadyState type is a string

[MattiasBuelens]

MediaSource.readyState is a ReadyState string enum, but TypeScript reports it to be a number.

[msftclas]

Hi @MattiasBuelens, I'm your friendly neighborhood Microsoft Pull Request Bot (You can call me MSBOT). Thanks for your contribution!

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[msftclas]

@MattiasBuelens, Thanks for signing the contribution license agreement so quickly! Actual humans will now validate the agreement and then evaluate the PR.

Thanks, MSBOT;

[zhengbli]

LGTM 👍
Thanks for contributing @MattiasBuelens !