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.