feat(virtual-core): add startItem option

[Dsaquel]

🎯 Changes

The goal of this pr is to give the possibility to add a custom left or top for a virtual item.
Typical scenario: Having a horizontal timeline where every width can be a place for a start item.

✅ Checklist

• I have followed the steps listed in the Contributing guide.
• I have tested and linted this code locally.
• I have generated a changeset for this PR, or this PR should not release a new version.

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Latest commit: 65a81e4

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