Add a toggle to make brave instances per-instance instead of per-tab in private mode

[coolanybody]

I use brave a lot on my phone because it’s simple enough to use and it’s safer than safari, plus the adblocking is more strict. Because of privacy, I like to use private-only mode so it removes all my logins after I close the app. However, I don’t like how now even opening a separate tab, even if it branches from one tab, logs me out of all the services I was logged in to on the other tab. This is extremely annoying, especially for some things I use that require me to be logged in to the website. I would like a way to make it so it logs out after closing, but every tab isn’t considered basically a new window. I’m asking to make it more similar to something like firefox or chrome’s private/incognito where everything is logged out after you close the window, not the tab.