refactor: improve API readability and tidy actions submodules (#2593)

* refactor: improve API readability, tidy actions modules

* Apply requested changes

* `actions/reloaders/reloaders.lua` -> `actions/reloaders.lua`

---------

Co-authored-by: Alexander Courtis <alex@courtis.org>

lua/nvim-tree/actions/tree/modifiers/init.lua

@@ -0,0 +1,11 @@
+local M = {}
+
+M.collapse_all = require "nvim-tree.actions.tree.modifiers.collapse-all"
+M.expand_all = require "nvim-tree.actions.tree.modifiers.expand-all"
+M.toggles = require "nvim-tree.actions.tree.modifiers.toggles"
+
+function M.setup(opts)
+  M.expand_all.setup(opts)
+end
+
+return M