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...

まず issue #6658 を読み、現在の JavaScript 診断の動作を確認してください。メモでは、TypeScript 構文の扱い、エラーと警告の区別、プロジェクトで設定可能な警告が、範囲を定めるべき領域として挙げられています。実装を完了と見なすには、まず設計を確定させる必要があります。