Support SemverJ as Version manager

[wolframhaussig]

closes #440

• added dependency semver4j
• added SemverJVersionManager to prevent breaking change for existing users (default implementation is still DefaultVersionManager => explicit opt-in required)
• added isStable to VersionManager interface making it easier to only allow snapshots in test environments -> just override AbstractPluginManager.isPluginValid
  • default implementation in interface returns always true to not break existing implementations
• added test method for DefaultVersionManager for snapshots
• added tests for SemverJVersionManager

[decebals]

It's a good PR but for the moment I don't know what to do.
I see that the latest commit on SemverJ was on September 2020 (something in readme). I am afraid that the project will be inactive.
Probably will be a good idea to introduce SemverJ as optional dependency at runtime.

[wolframhaussig]

I have updated my PR to make the library optional.

Another thought - untested yet: What would you think of using the official maven library: the latest version is from april 2021 and we would always be compatible with the maven versioning.

From what I have found in the javadoc you can:

• parse versions
• compare versions
• check versions against restrictions

some example of maven version ranges taken from the javadoc:

• 1.0 Version 1.0
• [1.0,2.0) Versions 1.0 (included) to 2.0 (not included)
• [1.0,2.0] Versions 1.0 to 2.0 (both included)
• [1.5,) Versions 1.5 and higher
• (,1.0],[1.2,) Versions up to 1.0 (included) and 1.2 or higher

[wolframhaussig]

I have added maven as alternative VersionManager - have a look and tell me what you think...