[native] Prevent text from being highlighted on press when if is no pressable

Summary:
We want to only define an onPress handler in MarkdownParagraph if there is a pressable in the message.

Context: https://linear.app/comm/issue/ENG-2389/prevent-text-from-being-highlighted-on-press-when-if-is-no-pressable

Test Plan:
The before is attached in the Linear post, and I will attach the after video below.

Reviewers: atul, ginsu, ashoat

Reviewed By: ashoat

Subscribers: ashoat, tomek

Differential Revision: https://phab.comm.dev/D5794