v4.0.0: Go module compatibility

This release introduces the new v4 directory as the base for all v4 versions.
This change makes all v4 versions go module compatible.

For stability reasons, the root of the repository keeps all v1-v3 versions but will only receive bug fixes.

If possible upgrade to v4, it is fully backward-compatible with v3, but resides in its own module.

go get github.com/blang/semver/v4@v4.0.0
...

import github.com/blang/semver/v4
v1, err := semver.Make("1.0.0-beta")
v2, err := semver.Make("2.0.0-beta")
v1.Compare(v2)

Thanks for all the contributions and the patience for this release