fix(VCarousel): add keys to delimiter buttons

[Tofandel]

Motivation and Context

The missing key would create issues with SSR, the controls would in some rare random cases get duplicated

The vue doc does say it's required on templated v-for custom components, I don't know how much of that applies to render components but from some reading through vue's code and some debugging, I'm confident the same applies
https://vuejs.org/style-guide/rules-essential.html#use-keyed-v-for

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• Improvement/refactoring (non-breaking change that doesn't add any features but makes things better)

Checklist:

• The PR title is no longer than 64 characters.
• The PR is submitted to the correct branch (master for bug fixes and documentation updates, dev for new features and backwards compatible changes and next for non-backwards compatible changes).
• My code follows the code style of this project.

[Tofandel]

Ideally I'd use something unique in this.items[i], but except the internal _uid of the component there doesn't seem to be anything else to identify it, but given that this is not using transitions and that if the items change the render gets invalidated, using the loop index is fine