Pre-suppress unsafe string key access errors in xplat/js (#44221)

Summary:
Pull Request resolved: https://github.com/facebook/react-native/pull/44221

This diff pre-suppresses errors of the following pattern, to prepare for the next Flow release.

```
declare const obj: {foo: string};
declare const key: string;
obj[key]; // error: invalid-computed-prop
```

Changelog: [Internal]

Reviewed By: alexmckenley

Differential Revision: D56477899

fbshipit-source-id: 5676b8685bd3157a519fe433cfce0fa28e003502