[Vue] Add type inference for Story and Variant slots item state, based on initState

[medfreeman]

Clear and concise description of the problem

As a developer using Histoire I want to add generic type inference to Story and Variant components so that when the initState property is specified, it is reflected into both default and controls slot items state property.

(while preserving the default type of any or Record<string, any> if initState is not specified)

PR: #623

Suggested solution

PR: #623

Alternative

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Additional context

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