Uses kas-gui/kas-text#132, replacing the old Effect token with two new token types: Colors specifies foreground and background color while Decoration specifies underline and strikethrough effects.

Decorations are drawn independently, which in theory allows multiple simultaneous decorations of different colors (in practice nothing uses this capability yet, and I think only something like generic HTML support would do so in practice).

Text selection/highlighting effects uses the new color tokens, replacing old special-purpose code. This is designed to allow highlighting which specifies background colors, though combining this with text selection will require modifying the token sequence at draw time.

Copies trait FormattableText and the Markdown parser over from kas-text since these now use kas-core specific effect tokens. For now, the old versions still live in kas-text.