### Description

Step 3 in our chunking refactors extracts `ChunkItem::as_chunk` into a
new `ChunkType` trait. This new trait isn't useful yet, but will
eventually allow us to collect `ChunkItems` of a similar type into a
lists, and perform chunking over all similar items at once.

Re: https://github.com/vercel/turbo/pull/6120

### Testing Instructions



Closes WEB-1722