Refactors Diagnostics so ALL diagnostics need to be registered through DiagnosticManager , publically available through Program

DiagnosticManager has the following features:

• a single diagnostic can be attached to multiple contexts
• contexts optionally include scope, tags or particular Ast Nodes
• if the same diagnostic is registered with a different scope context, it is added a related information
• individual diagnostic contexts can be cleared - if all the contexts are cleared for a diagnostic, then the diagnostic is automatically removed.