Essentially a minimized binary-compatible version of #3356, renaming all the factory methods for various tasks to follow more standard naming conventions:

• T {...} -> Task {...}
• T.command {...} -> Task.Command {...}
• T.input {...} -> Task.Input {...}
• T.source {...} -> Task.Source {...}
• T.sources {...} -> Task.Sources {...}
• T.persistent {...} -> Task.Persistent {...}
• T.task {...} -> Task.Anon {...}

The type T[_] remains an alias for Target[_], and Task{ ... } returns a T[_], to maintain binary compatibility. Not quite ideal but can probably be hand-waved away until Mill 0.13.0 when we are allowed to break binary compatibility.

All the T.* operations have been duplicated to Task.* by sharing them via a trait TargetBase, except the factory methods which were copied over and upper-cased while the old version deprecated. I have updated all the code and examples to use Task instead of T where relevant. The only exceptions are the implicit def applys which needed to be manually copied without the implicit (otherwise the multiple implicits cause ambiguity).

This gets us most of the user-facing benefits of #3356 without the bin-compat breakage: users no longer see an odd T { ... } syntax in the docs and in their build files, and now see Task { ... } which should be much more familiar. Although it does not allow us to do the type-hierarchy cleanups that the other PR provides, it's still worth doing so we can get it in in 0.12.0

The old T { ... } and T.* syntaxes should continue to work, and are exercised via the bootstrap tests as they continue to be used in Mill's own build. This PR should be source compatible to avoid migration pains, and given the prevalence of T everywhere we probably should just support it forever