Add next and previous methods to javascript api

As it stands, the javascript API has the following methods for querying elements:
- `find`
- `findAll`
- `closest`

These methods match some of the custom selectors that are supported by the [hx-target attribute](https://htmx.org/attributes/hx-target/).

It would be handy if the remaining selectors (`next` and `previous`) were made available through a public api in the same way that `find` and `closest` are made available.

There seem to be functions that already exist for this ([`scanForwardQuery` ](https://github.com/bigskysoftware/htmx/blob/e4f8fe9a7719533074d397446200a2795fe9a7ad/src/htmx.js#L1205)and [`scanBackwardQuery`](https://github.com/bigskysoftware/htmx/blob/e4f8fe9a7719533074d397446200a2795fe9a7ad/src/htmx.js#L1221)) but they are not public and therefore cannot be used.