FR: Allow `box()` to access distant modules through a URL

### Please describe your feature request

Hi,

Thank you for this incredible package!

I mostly use `box` to share small sets of documented functions when creating packages would be overkill.

In that context, it would be tremendously handy if one could point to a URL.
I could use `devtools::source_gist()`, but `box` offers the great advantage of documenting its functions.

For instance, I'd love to be able to write something like the following:
```r
url = "https://raw.githubusercontent.com/klmr/box/b386bac4d7269f989f63c1fd650e8be464dd44ed/inst/mod/hello_world.r"
box::use(world = url)
```

PS: the link to your contributing guidelines is 404 dead.

Start with the example URL and `box::use(world = url)` shown in the issue, then identify how this request fits box’s existing documented-function behavior. Done should be defined as loading a distant module through a URL while preserving that behavior. The contributing-guidelines link is also reported as a separate 404 issue.

Written by the indexing model from the issue text.