feat: add config option to set gui style

[riddlew]

Currently, the gui style of both fg and bg highlight groups are hardcoded so that fg defaults to gui=NONE, and bg defaults to gui=BOLD. This prevents you from being able to customize the gui style, such as using bold,underline with the fg style.

This PR adds the ability to control the gui style of both the foreground and background highlight groups using the normal gui=... syntax, such as:

  gui_style = {
    fg = "NONE", -- No gui style for fg highlight group
    bg = "bold,underline", -- Use underlined bold style for bg highlight group
  },