Added session-tab icon numbers

[bradleyrichter]

Test plan

1. Launch Brave, right click on the +, and create a new session tab (session 1)
2. Repeat for a few more sessions
3. Verify that each has the number shown
4. In each tab, navigate to a page and use middle click (or cmd + click) to open a link in a new tab
5. Verify the link which opened in a new tab has the same number shown as the opener

Automated test plan

npm run test -- --grep="should show partition number for new sessions"
npm run test -- --grep="should show max partition number even if session is bigger"

Original issue description

As a user, I need a way to differentiate session tabs at a glance.

Each tab has a session ID associated with it. The corresponding session tab number will be shown with a matching icon from 1-9.

Later, if more session numbers are necessary, we will switch to text-string numbers.

session_tab_icons.zip