feat(highlighting): support array syntax for nested attributes

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Summary
Allow attribute key to receive a string[] containing the path of the highlighted element in the hit.

TypeScript
I couldn't find a good way to implement deep object pathing, so it's not typed for now. It also looks like that a PathOf type is
still discussed.

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[francoischalifour]

Can we add a test with an attribute that contains a . to make sure that it works? It's a recurrent problem for our customers.

Example

const hit = {
  translations: {
    'fr.EN': 'Muggle',
    'fr.CA': 'Muggle',
    'fr.FR': 'Moldu',
  }
}

highlightHit({
  hit,
  attribute: ['translations', 'fr.FR']
})

A successful test and a test that warns would be great!

[francoischalifour]

Nice–good to go with these last changes!