feat: add poll shape

[danielpetri1]

When BigBlueButton displays a poll on the canvas, the client currently extends Tldraw with a custom shape. As a result, only the Tldraw instances tied to bbb-html5 profit from the functionality, despite other components, such as bbb-playback, bbb-export-annotations, and bbb-presentation-video needing to display such a poll.

This PR is a stepstone towards centralizing the poll shape directly in Tldraw, which would close the following issues:

• Missing polling result annotation in BBB 3.0 circa alpha.7+ bbb-presentation-video#52
• Annotation poll result missing in 3.0 circa alpha.7 bbb-playback#277

Development plan

1. Add the custom shape to the Tldraw fork

2. Change BBB 3.0 core to include the poll shape data in tldraw.json for bbb-playback
   2.1. This automatically makes the new polls shapes appear in the recordings

3. Remove duplicated custom shape from BBB's client, using the new implementation instead (we are here)

4. Change Tldraw to include an image representation in the shape's meta parameter

5. Use the poll image in the annotation export

6. Use the poll image in the video

For the poll image, we can follow the same approach used by the slide snapshot: bigbluebutton/bigbluebutton#21005

Demo

tldraw.polls.mov