fix: render Slices in the correct order in <SliceZone>

[angeloashmore]

Types of changes

• Chore (a non-breaking change which is related to package maintenance)
• Bug fix (a non-breaking change which fixes an issue)
• New feature (a non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)

Description

Related issue: #101

This PR fixes an issue where Slices could render in the wrong order in <SliceZone>.

The bug only occurs when the list of Slices changes after the first render. The bug occurs because the key prop is generated using the Slice's object. If multiple Slices contain the same payload (e.g. same Slice type, content, etc.), they would have duplicate keys.

To reduce the possibility of key collisions, this PR appends the Slice's index to the key.

Checklist:

• My change requires an update to the official documentation.
• All TSDoc comments are up-to-date and new ones have been added where necessary.
• All new and existing tests are passing.

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