Add color space support for 2D canvas and ImageData

This work was previously developed in https://github.com/WICG/canvas-color-space/blob/main/CanvasColorSpaceProposal.md and discussed in https://github.com/w3c/ColorWeb-CG.

This does not yet tackle ImageBitmap or ImageBitmap rendering contexts, as that needs a more holistic approach; see the discussion in #6562 (comment).

This also includes a lot of editorial modernization to the touched-upon sections of the canvas and ImageData spec text.