Support compiler/language services in .NET Core builds

[pdeligia]

The rewriter/compiler (from the high level P# syntax to the C# library version), as well as the language services and static analysis projects in general, are not ported yet to .NET Core. This is because some APIs (related to msbuild) are missing in .NET Core, but should be available probably in the next release.

We should investigate (temporary) replacements for these APIs, or wait for them to become available, depending on how soon we need to support the language services.