Integrate Cosmic Text

[ealmloff]

Specific Demand

Cosmic text implements some more advanced text layout and editing. It supports rendering bold, italicized, and multiline text. It also implements text editing on Unicode characters.

Implement Suggestion

Replace the current text layout engine with Cosmic text and use Cosmic text to implement editing when input widgets are implemented.

Related: https://github.com/linebender/vello/issues/306