Enable JavaScript specific warning behavior

See #6658 for background. Currently for JavaScript files we provide a very limited set of errors, mostly for syntax errors. There are a number of grammatical and type errors that would be useful to display also. Brief notes from ad hoc discussion in the team room include: - Move the code that flags TypeScript syntax in JavaScript files for better error handling - Add a distinction between errors and warnings to avoid "valid" JavaScript containing errors - Make the warnings user configurable as part of the project (i.e. may enable/disable certain warnings) More detailed design to come...