Switch text preview to range reads

Alex noted how partial reads done during preview of text files may lead to leaking resources https://github.com/ipfs-shipyard/ipfs-webui/pull/1646#issuecomment-699043284

The whole point here is that we want to avoid unnecessary reads because text files such as logs can be pretty big.
Draining the iterator until completion when a text file is tens/hundreds of megabytes may not be the best approach to ensure resource cleanup happens.

Perhaps could leverage the fact that `ipfs.files.read|cat` support range requests via `offset` and `length` parameters?

The idea is that if the preview ask for specific byte range (256KB block), it no longer needs low level handling of iterator. Something like ` await concat(await read({ offset: n, length: 262144 }))` will read 256K block and without leaking any resources.