fix(build): distinguish vim.mpack from global require'mpack'

problem: the api of vim.mpack is not compatible with a system provided mpack
solution: don't require 'mpack' directly from the system path

scripts/genvimvim.lua

@@ -1,4 +1,4 @@
-local mpack = require('mpack')
+local mpack = vim.mpack
 
 if arg[1] == '--help' then
   print('Usage: lua genvimvim.lua src/nvim runtime/syntax/vim/generated.vim')