feat: launch arg to start logged in as a supplied user

[lbrooney]

Add a new optional launch arg , --login <username>, that allows a user to specify which account they want to start Chatterino logged in as. If an account with the supplied username does not exist, then it starts anonymous. This arg is essentially equivalent to modifying current in settings.json before launch.

I tried to implement similar to how channel args are applied in WindowManager by applying the username in TwitchAccountManager, but I don't know if it would be preferred to elevate it to AccountController instead. I did not implement the platform specificity that the channel and activate options have.

I don't know if this is worth noting, but a side effect is that if the user never switches accounts, the current saved in setting.json will be set to whatever username they provided.

[github-actions]

clang-tidy made some suggestions

[pajlada]

Great first PR - thank you!

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