Name of position and position_type properties doesn't match CSS spec

[nicoburns]

Problem

• What we call position_type is Taffy is position in the CSS spec https://developer.mozilla.org/en-US/docs/Web/CSS/position
• What we call position is inset in the CSS spec. https://developer.mozilla.org/en-US/docs/Web/CSS/inset

Context

I suspect inset hadn't yet been specified when Taffy/Stretch was first created.

Proposal

Rename position to inset and position_type to position to align with the CSS names for these properties.