Added get_process_id / get_thread_id

I noticed that the `pid` and `tid` values that this plugin writes out seem to just be placeholders. I've added implementations for the events that have been working for me and wanted to contribute them back. It's worth noting that `get_thread_id` relies on FFI, which is a Luajit only feature. If Neovim runs from non-luajit then we return the old placeholder thread ID, `1`. That said I think just about everyone using Neovim uses the default lua version which is luajit. So this case should be pretty rare.