Change Dockerfile to install lua, luajit and love and also to install
nodejs 12 needed to build the lua debug adapter. Create the
love-headless test in support/test/lua to test love without an x server.
Sometimes it can take quite a while to start up and initialise the debug
adapter. So we use popup/float to display the status as we start up and
shut down.
This increases minimum Vim version to 8.2, but that's been out for ages
now and I intend to agressively require latest/later vim/neovim
versions.
