dsh-base64url: Base64URL Encoding and Decoding in DSH

The DSH plugin `dsh-base64url` provides Base64URL encoding and decoding in Node.js environments without requiring network services. To install it, run `dsh plugin add` and enable it in the profile configuration. It depends on specific versions of `@deepseek-ai/cordis` and `@deepseek-ai/dsh-tools`. Installing it executes third-party code, so please review the source code on your own.