[native] Refactor markdown-link component

Summary:
This diff extracts markdown-link.react.js code to markdown-utils.js. This code is going to be reused in markdown-chat-mention component.

Depends on D8834.

Test Plan: Tested later in the stack.

Reviewers: tomek, inka, rohan

Reviewed By: inka

Subscribers: ashoat, tomek

Differential Revision: https://phab.comm.dev/D8844