[RNMobile] Gallery - Native gallery component draft

[mkevins]

Description

This PR introduces a native Gallery component for the semi-cross-platform Gallery block. The purpose of this component is to render a list of images passed via the images prop.

PR Hierarchy

This PR is part of the PR hierarchy described here. This PR can be tested with the aggregate changeset and integration of components within the related PRs by checking out the branch of the "top level" PR: #18265 via the gutenberg-mobile PR: wordpress-mobile/gutenberg-mobile#1498.

MediaPlaceholder

Changes to MediaPlaceholder from this PR: #18262 have been merged for review here. The MediaPlaceholder component has been modified to append media for the native Gallery implementation.

On web, when new media is added to a gallery, the current selection of items in the gallery is available to the media selection interface, and can be presented as such to the user. The selector behaves more like a "gallery editor", in the sense that the selection is modified (users can add and / or remove items). On mobile, the interface for selecting media items is handled by the "parent apps", and does not "know" about what is currently selected as part of the gallery.

This PR adds the addToGallery prop to the MediaPlaceholder component, and conditionally appends media items to the current selection, passing the resulting collection back through onSelect when the addToGallery flag is set. When the addToGallery flag is not set, onSelect behavior is not modified.

Note: In this implementation, duplicates are removed from the collection via the id property, since this is used as a key in the list of elements that renders these items.
The MediaPlaceholder

The changeset here is used in the related "parent" PR: #18111, which includes these changes integrated with related changes in other components necessary for the gallery block.

Block-level caption

This component uses the mobile Caption component to add a block-level caption for gallery.

To test

Test this component by checking out the branch of the "top level" PR: #18265 via the gutenberg-mobile PR: wordpress-mobile/gutenberg-mobile#1498.

Checklist:

• My code is tested.
• My code follows the WordPress code style.
• My code follows the accessibility standards.
• My code has proper inline documentation.
• I've included developer documentation if appropriate.