chore: release v4.2.1

[Krinkle]

• expand peer dependency to include QUnit 3.0 support. This is reverse-integrated upstream at https://github.com/qunitjs/qunit/tree/3.0.0-alpha.3/demos/karma-qunit ... where it is passing with the new version. This allows projects to start experimenting with QUnit 3.0 alpha releases, without getting a warning from npm-install like:

  Could not resolve dependency: peer qunit@"^2.1.1" from karma-qunit@4.2.0 node_modules/karma-qunit dev karma-qunit@"4.2.0" from the root project