Use native 'base64url' buffer encoding

#### Support plan

* *is this issue currently blocking your project?* (yes/no): no
* *is this issue affecting a production system?* (yes/no): no

#### Context

* *node version*: v15.14.0
* *module version*: 5.0.0
* *environment* (e.g. node, browser, native): node
* *used with* (e.g. hapi application, another framework, standalone, ...): standalone
* *any other relevant information*:

#### What problem are you trying to solve?

Enabled more efficient `B64.base64urlEncode()` encoding by using the `Buffer` `'base64url'` encoding that [has been introduced](https://github.com/nodejs/node/pull/36952) in v15.7.0.

The feature can be detected by calling `Buffer.isEncoding('base64url')`.

#### Do you have a new or modified API suggestion to solve the problem?

No API changes required.