Add major version property method to all univers versions that don't already have this

In connection with work on a [VulnerableCode issue ](https://github.com/nexB/vulnerablecode/issues/1228) involving affected vs. fixed package versions, I realized that we need to identify and compare the respective major versions of a wide range of package types, namespaces etc.

Some univers versions already have property methods for major, minor and patch (e.g., `SemverVersion` and `ConanVersion`) while most others (e.g., `MavenVersion`, which inherits from `Version`) do not. It would be useful to add at least a major version property method (or equivalent approach) to the `Version` class to facilitate the sorts of major version comparisons mentioned above.