Add text staggering for overlapping labels and annotations

[cvanelteren]

Overlapping text boxes and annotations are a common pain point in dense figures. UltraPlot should provide an explicit helper to stagger existing text() and annotate() artists without changing their default behavior.

A useful API would support both subplot-level and figure-level usage, apply offsets independently within each subplot, and allow horizontal, vertical, or two-dimensional staggering so users can reduce collisions without manually tweaking every label.