Convert unannotated variable declaration with cast init to annotated variable declaration in xplat/js (#42880)

Summary:
Pull Request resolved: https://github.com/facebook/react-native/pull/42880

Convert `const x = (a: T)` into `const x: T = a`. These are equivalent in Flow, and helps reduce the amount of colon-casts.

```
js1 flow-runner codemod flow/castToAnnotatedVariable --target colon xplat/js
```

Changelog: [Internal]

drop-conflicts

Reviewed By: SamChou19815

Differential Revision: D53392999

fbshipit-source-id: 3b4602618d6990e8642b423edaeb4e89b01a199a