lua: make vim.cmd an alias of vim.api.nvim_exec() (#14401)

Previously vim.cmd was an alias of nvim_command().

From now on it is an alias of nvim_exec().

src/nvim/lua/vim.lua

@@ -309,7 +309,9 @@ setmetatable(vim, {
 })
 
 -- An easier alias for commands.
-vim.cmd = vim.api.nvim_command
+vim.cmd = function(command)
+  return vim.api.nvim_exec(command, false)
+end
 
 -- These are the vim.env/v/g/o/bo/wo variable magic accessors.
 do