Strict variance and read-only versions of types

I feel like we should have an open "suggestion" issue with a meaningful title to track the work on stricter variance and read-only versions of types that the TypeScript team is considering, especially as other issues are being duped against it. The initial design sketch and a few comments are in #17502.

After a little more searching, should this be merged with #10725?

まず #17502 の設計スケッチとコメントを読み、次にこの tracking issue と #10725 を比較してください。より厳密な variance と read-only types に関する作業をここで扱うべきか、それとも別の場所に統合すべきかを判断してください。作業の追跡先となる明確な issue が 1 つになれば完了です。