Refactor command naming convention into module dirs

[texastoland]

Stacked on #190 🚧 This PR uses module dirs to solve export use bugs documented in #184 (my previous attempt). Mostly mechanical changes. Enables the following:

use nu-git-manager gm                   # core gm commands
use nu-git-manager-sugar gm             # all gm commands
use gm [repo, gh]                       # shortcut commands

• [x] Add tests
• [x] Add modules
• [x] Add global gm export
• [x] Pass tests
• [x] Update readme
• [x] Rebase on main
• [x] Sanity test as my daily driver (most commands verified working)

[!NOTE] It stopped diffing git/mod.nu after git mv (I even tried fixing it in rebase) but I generated a diff using https://www.diffnow.com/report/mfg02.

[amtoine]

i will likely not land this right now... sorry for the delay :/

you're mentionning "bugs" in this PR, could you open an issue to document them, i'm not sure what you are talking about :innocent: