refactor(runtime): rewrite 'vim' color scheme in Lua

Problem: Bundled 'vim' color scheme is written in Vimscript which
  implicitly assumes that the file is ported from Vim.
  This is not the case, at it is currently the Neovim's way of providing
  backward compatibility for color schemes.

Solution: Rewrite it in Lua to indicate that this runtime file comes
  from Neovim.

test/functional/helpers.lua

@@ -41,7 +41,7 @@ module.nvim_argv = {
   -- Make screentest work after changing to the new default color scheme
   -- Source 'vim' color scheme without side effects
   -- TODO: rewrite tests
-  '--cmd', 'lua f=io.open("runtime/colors/vim.vim", "r"); l=f:read("*a"); f:close(); vim.api.nvim_exec2(l, {})',
+  '--cmd', 'lua dofile("runtime/colors/vim.lua")',
   '--cmd', 'unlet g:colors_name',
   '--embed'}