Port DOLFINx to Windows

[jhale]

This ports DOLFINx to Windows building against vcpkg using Microsoft Visual C/C++ compiler. It uses an overlay port for Intel MPI (MSMPI does not support neighbourhood collective operations from MPI3). Note that PETSc and petsc4py are not available on Windows.

The primary purpose of this port is to enable conda Windows packages, rather than users building DOLFINx on Windows from source themselves.

Main changes:

• Windows CI build of DOLFINx including C++ and Python unit tests and Python demos.
• Disables C99 _Complex when using Windows/MSVC.
• Allows for Basix to be found without asking Python module for hints -DDOLFINX_BASIX_PYTHON=OFF.
• Adds note about beta conda windows packages to README.
• Adds note about vcpkg windows build possibility to README.

Untested but potentially possible:

• Building against vcpkg on Linux and macOS.
• Building using a different compiler on Windows.

Outstanding points to tackle in other PRs:

• High runtime of Python unit tests - should be checked on Conda build. Probably slow JIT.
• Porting subset of C++ demos to work without PETSc.

[jhale]

Once this runs green it can be re-built on conda-forge. The basix branch should now be unnecessary, I removed the DLL stuff in favour of a proper solution.

Made all adjustments - let's see how running on PRs go then re-evaluate.