Use native 'base64url' buffer encoding

[kanongil]

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 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.