fix: support decoding data URL in Node < v16

[pastelmind]

I want to publish code that works on Node.js and web browsers alike, and I would like to support Node v14, which reaches EOL on April 2023. However, Node < v16 do not provide a global atob() function for decoding base64 strings, and must use Buffer.from() instead. (See docs for atob())

Furthermore, Buffer.from(str, 'base64').toString() is much faster than atob() in Node:

// Node v16.15.1

> var bigStr = btoa('1234567890abcdefghijklmnopqrstuvwxyz'.repeat(100000));
> console.time('atob'); atob(bigStr); console.timeEnd('atob');
atob: 117.196ms
> console.time('Buffer.from'); Buffer.from(bigStr, 'base64').toString(); console.timeEnd('Buffer.from');
Buffer.from: 9.797ms

This is because atob() is intentionally unoptimized. (See rationale)

Thus, we should prefer Buffer.from() whenever it is available.

This PR uses Buffer.from() if it is available, and atob() otherwise. In the rare event that neither exists, the code throws an Error.