Add a version constraint tester/checker to the view package sidebar

I think we should include the functionality of https://semver.mwl.be directly in Packagist. I'd imagine a little input box and stability drop down on the right side on a package detail page which then opens a layover with the selected versions and lets you try different constraints.

cc @hannesvdvreken

Start at the package detail page and its right-side sidebar, then compare the requested behavior with semver.mwl.be. Done means a version-constraint tester is available there, with a stability selector, a layover showing selected versions, and support for trying different constraints.

Written by the indexing model from the issue text.