Serialization of <position> argument of CSS circle() and ellipse() should match spec

[gsnedders]

The current serialization of the argument to CSS circle() and ellipse() was not following the spec, which outlines exactly how specified and computed serialization should behave.

This updates the implementation and quite a few tests to align.

WebKit-commit: https://commits.webkit.org/283287@main Bug: https://bugs.webkit.org/show_bug.cgi?id=279200 Reviewed-on: https://github.com/WebKit/WebKit/pull/33195

cc @weinig — can you check my merge resolution of css/css-shapes/parsing/shape-outside-computed.html and css/css-shapes/parsing/shape-outside-valid-position.html looks right to you? I'm mostly sus about some of the new tests that were added after your commit (especially around sign and sibling-index).