Replace my UI stuff with Plenary

[ckipp01]

Task

So looking at the docs fornvim_open_win() the config can just take a border argument. There is no title like I currently have, but it might simplify things a lot, which I'm all about. I sort of hate all the stuff I have in ui.lua, and I wouldn't mind literally replacing all of it if I could

Related to this, something else that has been on my mind is that there are certain things in nvim-metals that also part of https://github.com/nvim-lua/plenary.nvim. I think if we are going to start ripping stuff out and replacing it, it may be a good time to really go through everything and see what we could remove via plenary, and then just require plenary as a dependency.