clean sugar github (#31)

related to 
- #106

## description
this cleans up the `gh.nu` module into `github.nu` and will address one
task in #106

### new commands
- `gm gh query-api`: this is the biggest one which does all the API
heavy lifting
- `gm gh query-releases`
- `gm gh query-user`
- `gm gh pr checkout` inspired from old `me pr`

### removed commands
- all the old `gh.nu` module
- the now unused `fs/dir.nu` module

### tests
i don't think i'll add tests for that right now as the targets will be
moving 🤔