Bug fixes / New feature for countdown extension

[Shaobin-Jiang]

• Fixes the bug where countdown extension would not work properly in multiple trials
• Adds a new method for getting the amount of time left
• Adds a README

[changeset-bot[bot]]

🦋 Changeset detected

Latest commit: ce4e24758280ef604a986fd54fc2c28dba28ed9f

The changes in this PR will be included in the next version bump.

This PR includes changesets to release 1 package

| Name | Type | | ------------------------------------ | ----- | | @jspsych-contrib/extension-countdown | Minor |

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[jodeleeuw]

Thanks @Shaobin-Jiang !