This moves some stuff around to make LibGUI depend on LibSyntax instead
of the other way around, as not every application that wishes to do
syntax highlighting is necessarily a LibGUI (or even a GUI) application.
Rather than creating a TextStyle struct, and then copying its fields
over to a TextAttributes, let's just create a TextAttributes to start
with. This also simplifies the syntax highlighting code by letting us
define underlines along with the other text styling.
