bump Nushell and Nupm

[amtoine]

this PR

• bumps Nushell to 0.88.1
• bumps Nupm to https://github.com/nushell/nupm/commit/6a9618fa21453bca4000ac33707b7aace0da35c7 and https://github.com/nushell/nupm/pull/50

full changelog

• move the src/nu-git-manager/ module to pkgs/nu-git-manager/nu-git-manager/
• move the src/nu-git-manager-sugar/ module to pkgs/nu-git-manager/nu-git-manager-sugar/
• split package.nuon into pkgs/nu-git-manager/package.nuon and `pkgs/nu-git-manager-sugar/package.nuon
• fix all the imports, especially in the tests
• fix the install-package test of nu-git-manager
• clone the install-package test from nu-git-manager to nu-git-manager-sugar
• fix the listing of modules in the toolkit

TODO

• [x] fix the CI => https://github.com/amtoine/nu-git-manager/pull/135