Add version number constant

[jrfnl]

In select cases, it may be useful for a version number constant to be available to test suites integrating the polyfills.

Proposal:

• Add a VERSION constant to the Autoload class or a namespaced constant declared in the autoload file.

Would also be nice to add a CI check to safeguard that this is updated on every release, as that's easily forgotten.
Something like "for a PR to master, grab the top-most version number from the changelog.md file and verify that it is the same as the one used in the autoload file".

At the very least, the repo should start working with a release checklist which includes a check for version number being updated.