Short figure caption for outline

Description

Allow adding an optional short caption to be displayed in the outline

Use Case

When adding a caption to a figure, the text might be quite long, in particular, when the figure contains a table or (multiple) images that require explanation. The entire caption text is automatically added to the outline, which may look really messy, depending on the text length.
In the caption package of LaTeX, one can add an argument <list entry> (kind of a short caption), that shows up in the outline instead of the <heading> (i.e. the actual caption), if given: \caption[⟨list entry⟩]{⟨heading⟩}. If no <list entry> is given, the <heading> is shown in the outline (as is happening now in typst) and if it is given, but left empty ( \caption[]{⟨heading⟩}) , no entry will be made in the outline.

Start by locating the implementation of figure captions and the outline entry generation in the Typst compiler. Check how the existing caption text is passed into the outline, then verify that an optional list entry replaces it, an omitted entry preserves the full caption, and an explicitly empty entry suppresses the outline entry.