Local vcpkg.json support

[keynmol]

If the user has a vcpkg.json in their project, we should parse it and extract dependency list from it.

The dependency list should be merged with whatever is specified in the build tool.

• core - I don't think there's much we should do in here. I don't want to introduce any json dependency, and all build tools have some ability to read json.
• sbt-vcpkg - provide a vcpkgJson: Task[String], with ThisBuild / vcpkgJson := (ThisBuild/baseDirectory).value / "vcpkg.json", but also overridable in each project
• mill-vcpkg - something similar to sbt-vcpkg