Similar to `// @ts-ignore`, but will itself cause a new error diagnostic if it does not cause an existing diagnostic to be ignored.

Technical summary:
1. The scanner will now keep track of `CommentDirective`s it comes across: both `@ts-expect-error` and `@ts-ignore`
2. During type checking, the program will turn those directives into a map keying them by line number
3. For each diagnostic, if it's preceded by a directive, that directive is marked as "used"
4. All `@ts-expect-error` directives not marked as used generate a new diagnostic error

… ts-expect-error