feat(@clack/core): allow tab completion for placeholders

[JoshuaKGoldberg]

Fixes #114

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🦋 Changeset detected

Latest commit: 4f6fcf52f3e1cff3afe7b79c8b11e6bdc1753413

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

This PR includes changesets to release 1 package

| Name | Type | | ----------- | ----- | | @clack/core | Patch |

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