[nvim] Autocmd to highlight trailing whitespace using ErrorMsg

Additionally refactor the colors lua module into one with an init function that
init.lua can invoke.

nvim/init.lua

@@ -49,12 +49,14 @@ vim.cmd [[
 addGitTopLevelDirectoryToRuntimePath()
 
 require 'autocommands'
-require 'colors'
 require 'configuration'
 require 'diagnostics'
 require 'treesitter'
 require 'lsp'
 
+local colors = require 'colors'
+colors.init()
+
 local keys = require 'keys'
 keys.init()