Use path to indicate highlighted index of array items

[jsphstls]

Description

Highlighted items within objects clearly indicate the matching item by using a path:

"highlight": {
  "address.street": [
     "123 <em>Some</em> Street"
  ]
}

Yet, highlighted items within arrays do not indicate the matching item with respect to other items:

"_source": {
  "someArray": [
    "some item",
    "another item"
  ]
},
"highlight": {
  "someArray": [
     "<em>some</em> item"
  ]
}

This makes it difficult to determine which item of an array was actually highlighted without parsing the content.

One possible improvement would be to use a path to indicate highlighted index of array items:

"highlight": {
  "someArray[0]": [
     "<em>some</em> item"
  ]
}

[elasticsearchmachine]

Pinging @elastic/es-search (Team:Search)