feat: add support for passing encoder/decoder

[jacobheun]

This adds the ability to pass the lengthEncoder and lengthDecoder options for it-length-prefixed. I included tests for fixed encoding/decoding.

It's possible that the encoder could be passed to each individual write, but decoding from a reader doesn't currently support changing the decoder, so I made this a one time configuration when creating the wrap.

[mkg20001]

LGTM, just wondering if we should allow per-read options (or per-read to override global options)

Would be something like readLP (userOpts)

lpOpts = Object.assign(Object.assign({}, opts), userOpts || {})

[jacobheun]

We had talked about adding a decode.single method to it-length-prefixed which would make this a lot easier. With how it's currently setup I think you'd need to construct a new it-length-prefixed to avoid using the global decoder.

[mkg20001]

LGTM