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

[johnmhoran]

In connection with work on a VulnerableCode issue 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.