chore: Remove use of global vars + other cleanups

lua/symbols-outline/ui.lua

@@ -1,6 +1,4 @@
 local vim = vim
-local config = require 'symbols-outline.config'
-local symbol_kinds = require('symbols-outline.symbols').kinds
 local M = {}
 
 M.markers = {
@@ -55,8 +53,6 @@ function M.setup_highlights()
     )
   end
 
-  local symbols = config.options.symbols
-
   -- markers
   highlight_text('marker_middle', M.markers.middle, 'SymbolsOutlineConnector')
   highlight_text(