license: migrate from MIT to GNU GPL-3.0

Replace the MIT license with the canonical FSF GPL-3.0 (Version 3,
29 June 2007), preserving the project copyright notice. Add the SPDX
"GPL-3.0" license field to package.json, update the README license
line and add a GPLv3 badge, update CURRENT_STATE, and append a
changelog entry. GPL-3.0 blocks rebranding/closing the open-source
project as one's own while still permitting contributions, personal
use, and commercial redistribution with source disclosure.

Author: muhammadhasnainsaeed