[lib/native] introduce parseUserMentions function

Summary:
This diff introduces parseUserMentions. parseUserMentions takes the username found in the markdown entity/capture and uses that username to find the id of that user using memberMap

This is step 2 in the list below
Outlined below are the steps I will take to make @metion of a user into a touchable

1. Introduce createMemberMapForUserMentions function (Right now we only store a set of usernames but we also will want to know the id of the user based on the username)
2. Introduce parseUserMentions function (We will use this function to get and return a userID from the membersMap)
3. Introduce userProfileBottomSheetActive state to the markdown context (This will be used for disabling the press to focus on a message functionality)
4. Introduce MarkdownUserMention component (this will render the touchable that will navigate to the user profile)

Depends on D9391

Test Plan: Logged out the userID and confirmed that I was getting the correct value

Reviewers: atul, inka, rohan

Reviewed By: atul, rohan

Subscribers: rohan, ashoat, tomek, wyilio

Differential Revision: https://phab.comm.dev/D9392