Use asdf as a go library

### Is your feature request related to a problem? Please describe

Currently, we have an internal CLI tool (basically, to improve developer experience) at [jusbrasil](http://github.com/jusbrasil) written in go that has integration with `asdf` using inter-process communication (exec.Command). Since `asdf` was rewritten in go, I'd like to change this integration to use `asdf` as a go library instead but all the library code is in an `internal` package.

### Describe the proposed solution

I would like to have the `asdf` code library exposed so I can use it in another go program. To be able to do so, the code must be moved outside of the `internal` folder.

### Describe similar `asdf` features and why they are not sufficient

There is no similar feature in this case.

### Describe other workarounds you've considered

The workaround is using IPC for communicating with `asdf`.

Locate the asdf code currently under an internal package and review how its APIs are used by the CLI. The work is done when the library can be imported by an external Go program, allowing the described integration without IPC; verify the existing CLI still works.

Written by the indexing model from the issue text.