[lib/native/web] introduce createMemberMapForUserMentions function

Summary:
This mini stack right here will deal with making the @mention of a user a touchable that will navigate to the user profile whenever onPress is called.

This is step 1 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 D9390

Test Plan: flow and confirmed that there were no regressions with @mention of a user

Reviewers: atul, inka

Reviewed By: atul

Subscribers: ashoat, tomek, wyilio

Differential Revision: https://phab.comm.dev/D9391