[Core] Add redbot-update command for updating Red

[Jackenmen]

Feature request

Select the type of feature you are requesting:

• Cog
• Command (it's a cli command tho)
• API functionality

Describe your requested feature

I think it would be nice to have an easier option to update than running python3.7 -m pip install -U Red-DiscordBot[maybe_some_extra], it's long, not really something that a regular user would remember, the method of update differs a little on different OSes (executable name), it could just be easier to have a command like redbot-update (or if having something that simple in separate command is not good with you then perhaps redbot --update could work instead). I don't like redbot-launcher but that is one thing that is certainly easier for non-experienced user with it and we're gonna remove it at some point, I think it would be nice to have some good replacement for that functionality.

[palmtree5]

I'll look into this