Add config command

[alexarchambault]

This adds a config sub-command, that users can use to set and get values of pre-defined config keys, like

$ scala-cli config user.name "Alex"
$ scala-cli config user.email "alex@alex.me"
$ scala-cli config github.token "file:$HOME/.secrets/scala-cli-gh-token"
$ scala-cli config pgp.public-key
…

These values are meant to be used in the upcoming publish setup sub-command.

[lwronski]

I also have one question for this functionality. Is there an option to remove entry from ConfigDb?

[Gedochao]

if lines of commands in bash command blocks aren't starting with $ and the expected outputs are comments (starting with #), then it becomes possible to run the commands from the doc right in your terminal with a single click from the IDE, while browsing the .md file (screenshot below).

applies to the remaining code blocks as well.

[lwronski]

The only thing is missing for me, the --unset-all option, but that can be added in a separate PR