Restrict pointer to chart area

[mootari]

The pointer also responds to mouse movement outside the frame area. This can be disruptive when hovering over axis labels, especially when these labels have links.

Please consider adding an option to constrain pointers to a plot's frames.

https://github.com/observablehq/plot/assets/322014/f5d8838d-7f42-408c-9aaf-b0e8474b9386

[mootari]

Another side effect of this is that selecting text in an axis label will pin the tooltip:

https://github.com/observablehq/plot/assets/322014/24534f16-7915-4018-aa8f-cff197dcd028