Adjacent active chunks are handled as a single chunk

See this example: https://codesandbox.io/s/7j2njo9l2q.
```js
searchWords={["e"]}
textToHighlight="sleep"
activeIndex={0}
```
highlights both `ee` instead of just the first `e`. With `activeIndex={1}` nothing gets highlighted.

Start with the linked CodeSandbox reproduction and the searchWords, textToHighlight, and activeIndex props shown in the issue. Trace how adjacent matches are grouped and how activeIndex selects one, then verify that activeIndex 0 highlights only the first “e” and activeIndex 1 highlights only the second.

Written by the indexing model from the issue text.