Use `require('node:buffer')` rather than `Buffer` from global scope

Node.js recommends importing `Buffer` from `node:buffer`:
- https://nodejs.org/api/buffer.html#buffer
- https://nodejs.org/api/buffer.html#nodebuffer-module-apis

Using the `Buffer` from the global scope is causing issues for some users. E.g. https://github.com/spruceid/siwe/issues/168 and https://github.com/sveltejs/kit/pull/10214#issuecomment-1608991806