Release v4.0.0

[frederikprijck]

A new version of the library, including a couple of improvements:

• No longer include a polyfill for atob, as this is supported in all major browsers (and node environments > 14).
• Compile to ES2017, dropping support for anything that does not support ES2017 (which should be very limited according to caniuse)
• Use Node's atob when running on node.
• Drop support for Node 14 and 16, add support for Node 20.
• Add support for package.json's exports field, for better CJS/ESM support
• Reorganize build artifacts for better CJS/ESM support (cjs and esm needs to be their own directory with a cjs specific package.json file)
• Drop manual UMD bundle creation in index.standalone.ts, but rely on rollup instead.
• Infer JwtPayload and JwtHeader default types from the header argument by using overloads.

Even though some users might experience breaking changes, mostly because of the exports field, the majority should be able to update without making any changes, assuming the SDK is used in environments with support for atob.