Treat `T[keyof T]` as covariant (rather than invariant) during variance measurement

Suggestion

See title. Will fill in more details later.

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✅ Viability Checklist

My suggestion meets these guidelines:

• This wouldn't be a breaking change in existing TypeScript/JavaScript code
• This wouldn't change the runtime behavior of existing JavaScript code
• This could be implemented without emitting different JS based on the types of the expressions
• This isn't a runtime feature (e.g. library functionality, non-ECMAScript syntax with JavaScript output, new syntax sugar for JS, etc.)
• This feature would agree with the rest of TypeScript's Design Goals.

⭐ Suggestion

A lot of mapped types are covariant by construction, but we're not always able to determine that

📃 Motivating Example

💻 Use Cases

See incoming linked issues

Der Issue schlägt vor, T[keyof T] während der Varianzmessung als kovariant zu behandeln, und weist darauf hin, dass mapped types nicht immer als kovariant erkannt werden. Da keine Quelldateien, Tests oder Compiler-Einstiegspunkte angegeben sind, beginne damit, die Varianzmessung und die Behandlung von mapped types im TypeScript compiler zu finden. Als erledigt gilt die Aufgabe, wenn das beabsichtigte Kovarianzverhalten spezifiziert und durch geeignete Typprüfungs-Tests abgedeckt ist.