### Context
Task environment drivers must be moved to Framework projects so they can
be invoked from tasks. This is a prerequisite for the RAR task
enlightening and for fixing dotnet#13374.

### Changes Made
- Moved the task environment drivers to the Framework project
- Moved environment-related function from `CommunicationsUtilities` to
the Framework project
- Split in half `DebugUtils` and moved relevant part to the Framework
project. There is quite weird code path in
`CommunicationsUtilities.GetEnvironmentVariablesWindows` where it
ensures the DebugUtils static constructor is called because it sets the
environment variables that we would like then to propagate to other
msbuild processes.

### Testing
- [x] Unit tests
- [ ] Exp insertion

---------

Co-authored-by: Copilot Autofix powered by AI <175728472+Copilot@users.noreply.github.com>
Co-authored-by: Jan Provazník <janprovaznik@microsoft.com>

### Context
With the discovery of the "direct instantiation of the task" use case,
tasks must explicitly set a default task environment in order to work
correctly. Task authors may encounter the same issue, without having
benefit of having framework internals visible to them and thus without
ability to create one without re-implementing the driver themselves. So,
we need to provide a public API for `TaskEnvironment` construction to
enable setting the default task environment. Would be helpful for sdk as
well.

### Changes Made
Adds two public API members to `TaskEnvironment` so that task authors
using `IMultiThreadableTask` can obtain or create a `TaskEnvironment`
without depending on internal types:

- `TaskEnvironment.Default`: static singleton that delegates to the
process-level environment (multi-process mode).
- `TaskEnvironment.CreateMultiThreaded(projectDirectory,
environmentVariables?)`: factory method that creates an isolated
environment for multithreaded execution mode.

### Testing
Unit tests

### Note
Depends on #13380

### Context
With the discovery of the "direct instantiation of the task" use case,
tasks must explicitly set a default task environment in order to work
correctly. Task authors may encounter the same issue, without having
benefit of having framework internals visible to them and thus without
ability to create one without re-implementing the driver themselves. So,
we need to provide a public API for `TaskEnvironment` construction to
enable setting the default task environment. Would be helpful for sdk as
well.

### Changes Made
Adds two public API members to `TaskEnvironment` so that task authors
using `IMultiThreadableTask` can obtain or create a `TaskEnvironment`
without depending on internal types:

- `TaskEnvironment.Default`: static singleton that delegates to the
process-level environment (multi-process mode).
- `TaskEnvironment.CreateMultiThreaded(projectDirectory,
environmentVariables?)`: factory method that creates an isolated
environment for multithreaded execution mode.

### Testing
Unit tests

### Note
Depends on dotnet#13380