refactor `git` and `git-prompt` modules in `sugar` to allow testing

[amtoine]

related to

• https://github.com/amtoine/nu-git-manager/issues/24

description

the ./src/nu-git-manager-sugar/ directory structure changed from

.
|-- extra.nu
|-- git-prompt.nu
|-- git.nu
|-- github.nu
`-- mod.nu

to

.
|-- extra.nu
|-- git
|   |-- lib
|   |   |-- lib.nu
|   |   `-- prompt.nu
|   |-- mod.nu
|   `-- prompt.nu
|-- github.nu
`-- mod.nu

that is

• all the Git commands, e.g. gm repo ..., are still exposed as before from git/mod.nu
• git-prompt has been moved to git/prompt.nu and the main setup command is now called nu-git-manager-sugar git prompt setup
• the commands from past git-prompt.nu that would need to be tested have been moved to git/lib/ which is a private module