Avoid decoding to string before using papaparse

[LTLA]

V8 seems to have some limitation on the max size of the strings, which makes it unfeasible to fully decode a large file into a string. Rather, we can keep it as a Uint8Array and then wrap it in a File (browser) or ReadableStream (Node.js) before papaparse.

This does require a switch to a Promise-based framework, though, as papaparse uses callbacks when dealing with files.