Jar vcpkg.json support

[keynmol]

If we can find a vcpkg.json in some specified location in the jar of one of the dependencies, we should merge the configuration from that vcpkg.json with what we have locally.

Depends on #47.

After that, I believe we should provide some facilities for library authors to easier configure the two following things:

1. vcpkg-json-location attribute in META-INF/MANIFEST.MF in the jar, which points to the relative location of the vcpkg.json in the jar
2. the file itself

I believe just automating addition of attributes and vcpkg.json (copied from the location specified in the module)

• core nothing to be done I believe, apart from the constant for the attribute name
• sbt-vcpkg VcpkgLibraryPlugin which just does the things above
• mill-vcpkg not sure how best to handle it

[keynmol]

See

1. https://github.com/davenverse/sbt-npm-dependencies/blob/main/core/src/main/scala/io/chrisdavenport/sbt/npmdependencies/NpmDependencies.scala#L68 (collect manifests from classpath)
2. https://github.com/davenverse/sbt-npm-dependencies/blob/main/core/src/main/scala/io/chrisdavenport/sbt/npmdependencies/sbtplugin/NpmDependenciesPlugin.scala#L102-L113 (write manifest to a known location in the project)
3. https://github.com/davenverse/sbt-npm-dependencies/blob/main/core/src/main/scala/io/chrisdavenport/sbt/npmdependencies/sbtplugin/NpmDependenciesPlugin.scala#L83 (collect all manfiests from classpath)