feat(packer): query what plugin to load if none if specified

[AckslD]

This allows PackerLoad to take no arguments and in that case query the user for the plugin to load using vim.ui.select.

To do this a bit more cleanly I changed the command to be created with the lua api instead of vim.cmd. Maybe all commands should be changed to that but I think that's anyway the plan for #1042.

Also, is packer.loader considered part of the public api? Since this version changes it's expected argument. Could keep the same behaviour of packer.loader though if needed and have another internal function which is called by PackerLoad.

[AckslD]

I guess we need to check if nvim_create_user_command actually exists, since 0.5 and 0.6 are still supported.