[Modal] Add feature for setting the X/Y pos of Modal on the screen

[TatianaKapos]

Summary

Currently Modal just open on the top-left of the users screen. Upstream's modal is defaulted to the middle of the application.

First step is to default our Modal to popup in the middle of the application.

Motivation

Implementation of Modal

Basic Example

No response

Open Questions

Second step of this is to figure out if we want to add windows-specific properties to allow the Modal to pop/up anywhere on the screen (may be outside of the application boundaries which is not a use-case upstream has)

[chrisglein]

Some overlap with https://reactnative.dev/docs/modal#presentationstyle-ios, which is currently iOS only. That has some alignment options.

But as far as a default, centered would match expectation.