rename mise env-vars to mise set (and other env-vars improvements)

[jdx]

what do you think? (of course an alias would point back to env-vars for compatibility—this would just change the default name used in help and docs).

I think this is pretty clean:

$ mise set FOO=bar

I did recently add this alias:

$ mise ev FOO=bar

But I think I don't like it as much as set. This may be because I use fish and set is how you modify env vars so it's pretty natural. Perhaps bash/zsh users may not feel the same?

This is probably my favorite command that I suspect people rarely use. Perhaps it's on me to document it more clearly.

Some other improvements I want to see on this command:

• use ui::table when listing the vars (ex: https://github.com/jdx/mise/blob/main/src/cli/config/ls.rs)
• add mise set command
• add mise unset command
• add mise set --global flag
• make it possible to see a single var's value with mise set FOO
• add mise set instructions to environments.md