[native] memoized textStyle and styles.text

Summary: memoized textStyle and styles.text into markdownStyles for improved performance

Test Plan: Open a chat with any markdown components (blockquote, codeblock, etc) and everything should be the same

Reviewers: atul, abosh, rohan, kamil

Reviewed By: atul, rohan

Subscribers: ashoat, tomek, atul, abosh

Differential Revision: https://phab.comm.dev/D5251