refacto: abstract TreeExplorer in core.lua

lua/nvim-tree/core.lua

@@ -0,0 +1,26 @@
+local events = require "nvim-tree.events"
+local explorer = require "nvim-tree.explorer"
+
+local M = {}
+
+local first_init_done = false
+
+TreeExplorer = nil
+
+function M.init(foldername)
+  TreeExplorer = explorer.Explorer.new(foldername)
+  if not first_init_done then
+    events._dispatch_ready()
+    first_init_done = true
+  end
+end
+
+function M.get_explorer()
+  return TreeExplorer
+end
+
+function M.get_cwd()
+  return TreeExplorer.cwd
+end
+
+return M