CSS ray() function: Add "at <position>" feature row for Firefox116

[dipikabh]

Summary

Fx116 adds support for specifying initial starting position of the ray. The ray() function itself is still available behind the preference layout.css.motion-path-ray.enabled.

The new syntax:
ray( <angle> && <size>? && contain? && [at <position>]? )

This PR:

• adds the feature row "position" for at <position>.
• renames the "size-support" row to "size" for <ray-size>.
• renames <ray-size> to <size>: (update in content page)

Safari TP does not support <ray-size>, so removed the value for that feature row for Safari.

Test results and supporting details

Codepen: updated ray syntax (https://codepen.io/dipikabh/pen/yLReQaZ)

• Fx116 stable version: at <position> is supported behind pref and is optional.
• Chrome 116: at <position> is not supported even behind the flag Experimental Web Platform features (115).
• Safari TP (Safari 17.0): at <position> is not supported.

Related issues

Doc issue: mdn/content#27751
Content PR: mdn/content#28348

[dipikabh]

Thank you, @queengooborg 🙌