Add DartProject for Windows embedding API

[stuartmorgan]

This is a step toward aligning the API with macOS, and will make it easier to add the precompiled library later for release mode (since it can just be added to the project directory, without any code changes required for wrapper clients).

At the C API, uses a struct instead of individual arguments, mirroring a change that was already made on the Linux side to make the C API cleaner.

Functional changes in addition to the restructuring:

• adds relative path support, as was recently added for GLFW
• Uses wstring, rather than string, for paths; the conversion to UTF-8 is actually a potential problem on Windows, so pushing it into the embedding allows us the possibility of removing it later (if we can figure out a good solution at the embedder.h layer) without API breakage.

The old APIs used by the standard runner are left in place for now to avoid breaking the soon-to-land template on an engine roll. Once the framework template has been updated, the old API paths will be removed.