fix(lua): make vim.deepcopy work with vim.NIL

style: changed double quotes to single quotes

feat: add tests

fix tests

runtime/lua/vim/shared.lua

@@ -49,6 +49,9 @@ vim.deepcopy = (function()
     if f then
       return f(orig, cache or {})
     else
+      if type(orig) == 'userdata' and orig == vim.NIL then
+        return vim.NIL
+      end
       error('Cannot deepcopy object of type ' .. type(orig))
     end
   end

test/functional/lua/vim_spec.lua

@@ -419,6 +419,12 @@ describe('lua stdlib', function()
       return getmetatable(t2) == mt
     ]]))
 
+    ok(exec_lua([[
+      local t1 = {a = vim.NIL}
+      local t2 = vim.deepcopy(t1)
+      return t2.a == vim.NIL
+    ]]))
+
     matches('Cannot deepcopy object of type thread',
       pcall_err(exec_lua, [[
         local thread = coroutine.create(function () return 0 end)