Re-add Neovim configuration files

Re-included from https://github.com/opdavies/opdavies.nvim.

config/neovim/plugin/harpoon.lua

@@ -0,0 +1,13 @@
+require("harpoon").setup()
+
+local mark = require "harpoon.mark"
+local ui = require "harpoon.ui"
+
+vim.keymap.set("n", "<M-h><M-l>", ui.toggle_quick_menu)
+vim.keymap.set("n", "<M-h><M-m>", mark.add_file)
+
+for i = 1, 5 do
+  vim.keymap.set("n", string.format("<space>%s", i), function()
+    ui.nav_file(i)
+  end)
+end