Fix TypeScript types of `positions` in `GradientValue` (#53942)

Summary:
The original type of the `positions` property was mistakenly declared as a 2-dimensional array, which is not accepted in runtime. In reality, a flat string array is expected as per [linear gradient examples](https://github.com/facebook/react-native/blob/18ba21149ae46d3fa97fe0dcbe165999009d2a6e/packages/rn-tester/js/examples/LinearGradient/LinearGradientExample.js#L230).

## Changelog:

[General] [Fixed] - TypeScript types of `positions` in `GradientValue`

Pull Request resolved: https://github.com/facebook/react-native/pull/53942

Test Plan: N/A

Reviewed By: cortinico

Differential Revision: D83342637

Pulled By: javache

fbshipit-source-id: 556618dd43861b3d3f1728c089f790815f2f011b