Built-in task to show mill version

[rockjam]

There should be some embedded task to check mill's version. We could implement it as ExternalModule, available for all mill users like GenIdea module.

How to find out mill's version:

def millVersion() = T {
  System.getProperty("MILL_VERSION")
}

[lihaoyi]

version should probably be a built in task on MainModule rather than an ExternalModule, given it's so simple

[lihaoyi]

36eb8bd