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.
This highlighter just syntax highlights the commented lines in your git
commit message. It could potentially be enhanced to handle the rebase
UI or other more advanced cases in the future.
