Migrate file I/O tasks to multithreadable task API

[JanProvaznik]

This PR migrates several file I/O tasks to use the new multithreadable task API by implementing the \IMultiThreadableTask\ interface and using \TaskEnvironment\ for thread-safe path resolution.

Changes

The following tasks now implement \IMultiThreadableTask\ and use \TaskEnvironment.GetAbsolutePath()\ instead of \Path.GetFullPath():

• Copy.cs
• Delete.cs
• MakeDir.cs
• RemoveDir.cs
• Touch.cs

Why

\Path.GetFullPath()\ uses process-global state (current working directory) which is not thread-safe when multiple tasks run in parallel. The \TaskEnvironment.GetAbsolutePath()\ method provides thread-safe path resolution by using the task's execution context.

This enables these tasks to be safely executed in parallel when MSBuild's multithreading is enabled.