change license to unlicense as per original source

As this extension is based on Webfreak code-debug I think it would be reasonable to use the same license.
Note: this is not necessary as I've initially thought because the unlicense of the original repo actually allows license changes

Unlicensed works, modifications, and larger works may be distributed under different terms and without source code.

This PR adjusts the license according to https://github.com/WebFreak001/code-debug/blob/master/LICENSE and uses the same license identifier as "upstream" in package.json.

If this PR is not accepted the correct SPDX license identifier should be used in package.json (MIT?)