Add `AbsolutePath.StartsWith`

[ForNeVeR]

An API like AbsolutePath.StartsWith(AbsolutePath anotherPath) would be very useful.

[Komroncube]

what is the mission of this function? could you describe so I can imagine where to start

[ForNeVeR]

Its mission is to be a mirror of LocalPath::IsPrefixOf.

a.IsPrefixOf(b) should mean b.StartsWith(a), and it would be great to have it for both path types (i.e. implement as an extension method on Path<T>).

Motivation: in different contexts, the user may wish to express their logic via either method.

[ForNeVeR]

Closed by #69.