Add option to begin-at-new-page for (sub)tasks

[rabuu]

Describe your changes

This adds a begin-at-new-page option for the task and subtask functions.

It just inserts a colbreak before the task/subtask.
For the very first task, it does nothing. This enables the following pattern:

#let task = task.with(begin-at-new-page: true)

#task[ajakjal]
#task[ajkajl]
#task[aklak;lak;]

where every task begins at a new page.

Related issues and/or PRs

Resolves #63.

Checklist

• The test suite passes
• The code is formatted
• The code is readable and well-documented
• Every word that can be in the output is translatable
• All new features are reflected in a test or example
• All new examples have a generated associated test
• Changes to the public API are documented in the CHANGELOG
• All related PRs and issues are linked
• An appropriate label is chosen

[leanderhert]

Nice!
Looks greate, I have nothing to add.