Establish a Beta Release

Having a beta release should improve the Karma org health. Early community testing of breaking changes is a hallmark of mature projects. Also, it gives a chance for many breaking changes to be associated in a single release, when beta becomes stable. For now, for example, all the potential breaking changes are tracked very rudimentarily, in a [bug]; also the corresponding PRs ([example]) are tagged as breaking changes and left to sit until the time for a major release comes. With the beta release, all those changes could be tested and better managed and we eliminate the mess (authors disappear, changes become irrelevant, etc) of having unattended PRs.

[bug]: https://github.com/karma-runner/karma/issues/3503
[example]: https://github.com/karma-runner/karma/pull/3726