Add hint about how install missing plugins

[steveno]

While working on a project recently, I encountered the following situation:

steveno@shawnee:~/Projects/Project (main)$ mise current
mise [email protected] is specified in ~/Projects/Project/.mise.toml, but not installed
go 1.23.1

In https://github.com/jdx/mise/issues/2460, "hints" were added. This looks like the perfect place to add a hint. Something like,

steveno@shawnee:~/Projects/Project (main)$ mise current
mise [email protected] is specified in ~/Projects/Project/.mise.toml, but not installed
[mise hint] Install missing plugins using mise upgrade 
go 1.23.1