add: oil.lua to custom plugins

add oil.lua as custom plugin, a vim-vinegar like file explorer that lets
you edit your filesystem like a nomal neovim buffer

lua/custom/plugins/oil.lua

@@ -0,0 +1,20 @@
+return {
+  {
+    'stevearc/oil.nvim',
+    dependencies = { 'nvim-tree/nvim-web-devicons' },
+    config = function()
+      require('oil').setup {
+        columns = { 'icon' },
+        keymaps = {
+          ['<C-h>'] = false,
+          ['<M-h>'] = 'actions.select_split',
+        },
+        view_options = {
+          show_hidden = true,
+        },
+      }
+      -- Open parent directory in current window
+      vim.keymap.set('n', '-', '<CMD>Oil<CR>', { desc = 'Open parent directory' })
+    end,
+  },
+}