[native] Add a onLongPress handler in MarkdownLink

Summary:
We need to add a onLongPress handler in MarkdownLink that calls the onPressMessage method from MessagePressResponderContext so you can long press links to copy them.

Linear: https://linear.app/comm/issue/ENG-2452/add-a-onlongpress-handler-in-markdownlink

Test Plan:
See videos attached.

Before

After

Reviewers: ashoat, atul, ginsu

Reviewed By: ashoat

Subscribers: tomek

Differential Revision: https://phab.comm.dev/D5873