Add logical/abstract values for touch-action

[patrickhlauke]

Adds pan-inline, pan-inline-start, pan-inline-end, pan-block, pan-block-start, pan-block-end values for touch-action, and expands the explanation to reference https://www.w3.org/TR/css-writing-modes-3/

Closes #272

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Preview | Diff

[patrickhlauke]

Discussed at last PEWG meeting (31 Jan). There's a few more tiny language tweaks I'd love to make, just to make it clear which element's writing mode needs to be taken into consideration (tl;dr: the element where the touch-action is defined, and not any of the child elements inside it). setting back to draft until I sorted that.

[smaug----]

Removing needs-wpt for now, since this isn't even in the spec. We definitely should add a wpt at the same time when landing this

[patrickhlauke]

discussed in today's PEWG meeting. want to include a graph/chart

need to also clarify explicitly what happens if you have touch-action defined on an element, and then a child element defines a different writing mode/direction (in short, only the one on the element that defined touch-action counts)