plugins: add a plugin to manage terminal

- Adds the ToogleTerm plugin that allows to improve the managment of
  terminal buffers.
- Configures it.

Co-Authored-by: iGor milhit <igor.milhit@unige.ch>

lua/custom/plugins/toggleterm.lua

@@ -0,0 +1,16 @@
+-- A neovim lua plugin to help easily manage multiple terminal windows
+--   https://github.com/akinsho/toggleterm.nvim
+
+-- Set ZSH as shell to get it in a terminal buffer
+vim.opt.shell = "/usr/bin/zsh"
+
+return {
+  {
+    'akinsho/toggleterm.nvim',
+    version = "*",
+    opts = {
+      -- Avoid terminal to be darker than the Nord colorscheme
+      shade_terminals = false
+    }
+  }
+}