updating

[dragonwocky]

Currently, whenever a new version of Deno is released, there's no straightforward way to "update" to it.

• If you use the deno upgrade command, it works, but the new version isn't stored by dvm and it will be lost if you switch to another version of Deno.
• If you run dvm install to install and switch to the latest version, you end up with every patch version of Deno saved/installed. I don't mind keeping a v1.16.x and a v1.17.x installed, but I'm not all that likely to be switching back and forth between v1.18.2 and v1.18.1.

It'd be nice to have a dvm upgrade command that replaces the currently active installation with the latest installation as an alternative to the deno upgrade command.

On a related note, it may be worth adding a dvm upgrade self command that acts as an alias to curl -fsSL https://deno.land/x/dvm/install.sh | sh for updating the tool itself.