Introduce a concept of project solutions

[alexrp]

Basically a celerity.solution.json looking something like this:

{
  "projects": [
    "src/project-a",
    "src/project-b"
  ]
}

This file will indicate to various tooling (e.g. the language server) which celerity.json project files (#30) should actually be loaded as user projects. Without this mechanism, we have no good way to tell if a project file is only present because it comes from a git submodule-based dependency.