Open to a third argument to keep the elements to split at?

Sometimes you want to keep the element that causes the split (i.e., that the iteratee evaluates to true), i.e.,
```js
splitBy([1, 2, 3, "|", 4, 5, 6], val => val === "|", true); // [[1, 2, 3], ["|", 4, 5, 6]]
// ^^^^ NEW ^^^ NEW
```
Would you be open to accepting a pull request that implements this? The third argument could be called `keepSeparator` or something?

Start at the splitBy entry point and inspect how the iteratee currently determines boundaries. Implement and verify the third-argument behavior shown in the issue, ensuring the matching separator is retained at the start of the following chunk.

Written by the indexing model from the issue text.