refacto: move filters and sorters in their own modules

Also exclude filters is taken into account in git ignore
fixes #892

lua/nvim-tree/explorer/common.lua

@@ -0,0 +1,11 @@
+local uv = vim.loop
+
+local M = {}
+
+function M.has_one_child_folder(node)
+  return #node.nodes == 1
+    and node.nodes[1].nodes
+    and uv.fs_access(node.nodes[1].absolute_path, 'R')
+end
+
+return M