Feature: Point (pt) as new space unit for labels

Currently, there is space unit support for pixels (px) and the abstract world unit (world). The former is used for 2D Labels, the latter for 3D Labels.
Since px does not ensure an absolute size (i.e. the visible result depends on the display's dpi), pt would be useful to ensure an absolute size for labels, and thus, ensure labels' readability.

1 pt is 1/72 of an inch based on the physical size of the screen.

See [devicePixelRatio](https://developer.mozilla.org/en-US/docs/Web/API/Window/devicePixelRatio).

Start by tracing how existing px and world space units are represented and applied to 2D and 3D labels, then review the devicePixelRatio reference in the issue. Done means labels accept a pt unit and its physical sizing is handled consistently for readability.

Written by the indexing model from the issue text.