Multi Instance: Refactor: move core.get_cwd function into explorer

[alex-courtis]

• Multi Instance - Contributors Wanted - Please Take An Issue #2255

Independent of #2836

Some core functions operate directly on an Explorer instance and can be moved in there:
get_cwd

Some dependency cycles may be reduced after refactor e.g.

nvim-tree.lua/lua/nvim-tree/view.lua

Line 306 in a4699c0

local starts_at = M.is_root_folder_visible(require("nvim-tree.core").get_cwd()) and 1 or 0