Suggestion: Change `Diagnostic.file` to be `SourceFileLike` instead of `SourceFile`.

Diagnostic.file is of type SourceFile which forces plugins to cast a non-SourceFile to a SourceFile to report diagnostic messages and guess which parts of SourceFile are not used by tsserver.

Consider changing file to be something like SourceFileLIke or SourceMapSource that specifies the exact requirements that must be supplied by plugins for this field.

まず Diagnostic.file の定義と利用箇所を見つけ、次に tsserver が課す要件を SourceFileLike および SourceMapSource と比較します。最小限の plugin 向け契約について合意し、すべての diagnostic consumer にわたって公開型の変更を検証できれば完了です。