[lib] Update thread-utils to include specialRole field

Summary:
There are some places in the codebase that we hardcode isDefault. Here, we'll want to introduce the specialRole field alongside, so when we deprecate isDefault, no additional changes will be needed. Some are important (like one instance in thread-utils.js and the instance in role-creator.js where we set up initial roles for threads), and others are just test data. Nonetheless, I thought I may as well update all of them for consistency.

Part of ENG-5993

This diff handles thread-utils.js

Depends on D10108

Test Plan: Right now, just flow to make sure we didn't violate the type. Further testing can be done once isDefault is removed

Reviewers: ginsu, tomek, rohan

Reviewed By: tomek

Subscribers: ashoat

Differential Revision: https://phab.comm.dev/D10109