Add indices field to _matchesPosition to specify where in an array a match comes from (#5005)

* Remove unreachable code

* Add `indices` field to `MatchBounds`

For matches inside arrays, this field holds the indices of the array
elements that matched. For example, searching for `cat` inside
`{ "a": ["dog", "cat", "fox"] }` would return `indices: [1]`. For nested
arrays, this contains multiple indices, starting with the one for the
top-most array. For matches in fields without arrays, `indices` is not
serialized (does not exist) to save space.

crates/meilisearch/tests/search/formatted.rs

@@ -208,7 +208,10 @@ async fn format_nested() {
                         "doggos.name": [
                           {
                             "start": 0,
-                            "length": 5
+                            "length": 5,
+                            "indices": [
+                              0
+                            ]
                           }
                         ]
                       }