Truffle: add the ability to "tag" Sources with any number of standard or private tags, for example so that Sources might be marked as "FROM_FILE", "LIBRARY", "BUILTIN", or any other distinction that matters to some tools. Those tags can be applied by the language runtime when sources are created, for example when loading builtins. Alternately, you can listen for newly created sources from outside the implementation, where you might tag sources based on pattern matching against file paths or any other meta-information in the Source.