feat: support pull streams

[alanshaw]

This PR updates the normaliseInput function to accept pull streams.

I've also made the following changes:

1. Update the docs for supported inputs
   • Buffer|ArrayBuffer|TypedArray is aliased as Bytes
   • Blob|File is aliased as Bloby
   • Added info for what a input "means" i.e. causes single/multiple files to be added
2. Peek the first item of an (async) iterator properly
3. Move file object check below input[Symbol.asyncIterator] check because Node.js streams have a path property that will false positive the isFileObject check
4. Fix toFileObject to allow objects with no content property
5. Simplify toBuffer to remove checks that Buffer.from already does

[achingbrain]

If these inputs do actually result in multiple files (e.g. the collections have more than one entry):

Iterable<Bloby> [multiple files]
Iterable<String> [multiple files]
AsyncIterable<Bloby> [multiple files]
AsyncIterable<String> [multiple files]
PullStream<Bloby> [multiple files]
PullStream<String> [multiple files]

and they aren't added with the wrapWithDirectory option they'll cause the importer to return an error as they will result in multiple roots.

I guess we just let it fail there? Otherwise we have to validate the options too.

[alanshaw]

IIRC you can add multiple files as long as they're all in the same root...so we need these inputs for that at least. Multiple roots is interesting because I think that go-ipfs supports this and I know that we don't. I'm ok for the importer to error in this case until it can be fixed...