shapes: Use decoration style

[johannes-wolf]

Implements #465.

This PR is a WIP implementation on how to implement decorations as style option for all (but grid and mark) elements.

The idea:

• The context has a decorations dictionary that can be modified to add custom decorations.
  This allows for a function like register-decoration(...) for plug-ins (same plan as with custom marks; not added yet).
• The user can either use a decoration name (dict) or pass a function to decoration: .. for elements. Element paths get decorated before adding marks, which allows for styled paths to have marks.
• You can still use decoration-functions via composition.
• You can apply multiple decorations in sequence
• Decorations can be applied on a subsection of a path (start, end)
• This API will replace the current decoration lib
• It works without destroying marks

Demo of the wave and linearize modifiers:

[fenjalien]

We'll need to write a specification of what a decoration function should expect to receive and what it is expected to return.

What do you think of blocking this until we work on modifiers?

[johannes-wolf]

@fenjalien I've updated the API. This new modifier implementation is meant to completely replace the decortaions.path library and move path modifiers to cetz core.
I think decoration: should be named modifier:.

The PR is still very WIP but should give a first idea. It is a better implementation than what decorations.path currently provides.

I wrote some information to the connected issue.

[johannes-wolf]

Rebased.
How do we want to proceed with this?
I am open to change the name from modifier to something else, although I prefer modifier or decoration over show-rule, as it is more explanatory.

If we merge this, we can implement marks on top of this. I prefer not touching the mark implementation before we agree on a modifier/decoration implementation.

If we want to support outputting extra geometry from modifiers, we should discuss how to do that. The current interface does not allow this. To add support, the apply-* function must know if the returned drawable is part of a continuous path or if it is some “standalone” shape.

[fenjalien]

I'm happy to continue using modifier, but I think using show-* would help to draw a parallel to Typst's show rules. I don't like decoration.

I would really like to get on with this but I want to get the docs website up properly and I'm struggling to find time for it.