feat: add iOS support for CSS mask properties

[paradowstack]

Rebase Note:

This branch is stacked on #54995 + #54996 + #54994 (Nishan's url() in background-image work) and #58163 (JS & C++ changes). Once this work lands, this branch has to be rebased. Review only the last commit.

Summary:

This PR adds support for the mask-image, mask-size, mask-position and mask-repeat CSS properties. It follows the CSS spec described here. A mask clips a view and its whole subtree to the alpha channel of a gradient or an image. The remaining mask properties (mask-clip, mask-origin, mask-mode, mask-composite) and the mask shorthand are not covered here and can be added in a follow up PR if needed.

The mask layers are drawn with the same gradient and image helpers as background-image, assembled into a CALayer tree and assigned as the mask of the container layer, so masking is intersected with the border radius clipping shape and applies to children as well. url() masks reuse the background-image URL loader, so no new image loading code is introduced.

Work has been split into three stacked PRs for more convenient reviewing process:

1. JS - style attributes, Flow types and the cross platform prop plumbing in C++
2. this PR - iOS rendering and the RNTester example page
3. Android - Android rendering

The stack is based on the url() in background-image work, which has to land first.

Changelog:

[IOS] [ADDED] - Add iOS support for CSS mask properties

Test Plan:

Run RNTester on iOS and open the Mask screen (MaskExample.js) - gradient, local and remote image masks, mask-repeat, mask-size, mask-position, multiple layers, border radius, masked children, text, image, scroll view and text input.

[github-actions]

Warning

JavaScript API change detected

This PR commits an update to ReactNativeApi.d.ts, indicating a change to React Native's public JavaScript API.

• Please include a clear changelog message.
• This change will be subject to additional review.

This change was flagged as: POTENTIALLY_BREAKING