UI: Add a file input control #84181

Follow-up to #66403 and the discussion about adding a file control to the UI package.

What problem does this address?

The discussion in #66403 recommended adding a component for handling files to @wordpress/ui, rather than extending FormFileUpload in @wordpress/components. A reusable file input would give consumers a consistent way to select local files using the WordPress Design System.

There is no immediate need identified in that discussion, so this is a low-priority addition.

What is your proposed solution?

Add a file input type control to @wordpress/ui, styled with the design system's tokens. It should open the native file picker, let consumers access the selected files, and support native file input options such as accept, multiple, and disabled, with an accessible label and keyboard operation.

Use Base UI's Input as the upstream reference. The shadcn/ui Base UI file input example, which uses type="file", is the concrete counterpart cited in the original discussion.

Keep the initial scope focused on file selection. Drag-and-drop support, DropZone composition, and Media Library integration can be considered separately when a concrete use case needs them.