Rename extension add/remove to install/uninstall #501
Description
The commands for adding and removing extensions have been add/remove since they were introduced, however this is inconsistent with what most other tools like cargo/apt use and thus has popped up as frequent friction where install/uninstall would be tried intuitively until realizing it had to be add/remove instead.
To make sure that the correct command is picked intuitively most of the time, the subcommands should be renamed from add/remove to install/uninstall.
I've talked this through with @kylewillmon and @andreaphylum and 3/3 votes are in favor of install/uninstall. Which makes sense considering we work mostly with cargo.