Revert "Remove oil.nvim, configure netrw"

This reverts commit 94e6c8ab13.

after/plugin/netrw.lua

@@ -1,3 +1,6 @@
 vim.keymap.set("n", "<leader>pv", vim.cmd.Ex)
 
 vim.g.netrw_banner = 0
+vim.g.netrw_browse_split = 0
+vim.g.netrw_liststyle = 3
+vim.g.netrw_winsize = 20

after/plugin/oil.lua

@@ -0,0 +1,3 @@
+require("oil").setup {}
+
+vim.keymap.set("n", "-", "<cmd>Oil<cr>", { desc = "Open parent directory" })