Update to cosmic-text 0.15

[ickshonpe]

Objective

Update the cosmic-text dependency to 0.15

Solution

Very minimal changes here, nothing user facing except an error return type.

• Cosmic-text no longer uses ttf_parser, to validate fonts during asset loading a skrifa::FontRef is constructed instead. If the font is invalid, the font asset loader now returns a skrifa::ReadError, not a ttf_parser::FaceParsingError.
• To get a font from the global CosmicFontSystem instance, the get_font function requires a weight now. The weight is looked up from the CosmicFontSystems font database, immediately before the get_font call.