Let hillcharts move a list's position on the hill

`basecamp hillcharts` can show a chart and track or untrack lists, but it can't move a list's dot along the hill. We use the CLI to keep a project up to date as work happens: to-dos, cards, messages and chat all update automatically. The hill chart is the one piece we still have to update by hand in the web app.

It looks like the public API doesn't support this yet either. `sections/hill_charts.md` in bc3-api documents getting a chart and changing which lists are tracked (`PUT /todosets/:id/hills/settings.json`), but nothing for updating dot positions. So this might need an API endpoint first.

What would help:

```
basecamp hillcharts move --position 60 --in
```

Ideally it would also post a hill chart update with an optional note, the way moving dots in the web app does, so the change shows up in the project's activity.

Thanks!

p.s. I'm Fernando, from work, this is just my hobby Github account 😆

Start with sections/hill_charts.md in bc3-api and compare its chart and hill settings operations with the existing basecamp hillcharts CLI behavior. Determine whether an API endpoint is needed before adding the requested move command. Done means a list dot can be moved by position and an optional note can appear in project activity.

Written by the indexing model from the issue text.