Feature Request: add TypeChecker API to query if identifier is definitely assigned

Currently it's not possible to query CFA. I need to find out if an identifier is definitely assigned. This logic is part of checkIdentifier and is not accessible outside of that function.

My use case is the TSLint rule no-unnecessary-type-assertion where we need to identify false positives. The current implementation compares the type before and after the non-null-assertion. That doesn't work well for identifiers that are not definitely assigned, because the assertion suppresses TS2454 and can therefore not be removed.

まず、現在 definite-assigned check が含まれている checkIdentifier の制御フロー解析ロジックを読みます。TSLint の no-unnecessary-type-assertion ルールがその結果をどのように問い合わせる必要があるかを追跡し、公開 TypeChecker API の境界を定義します。呼び出し側が checkIdentifier の内部動作に依存せずに、識別子が definitely assigned かどうかを判定できれば完了です。