Multi Instance: Refactor: move nvim-tree.lua functions into Explorer

[alex-courtis]

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

Many nvim-tree.lua functions operate on an Explorer, move them in.

[alex-courtis]

Few functions could be moved.

The main usages of explorer are the autocommands. These could be moved into Explorer, with a group per instance, deleted on destroy.

[alex-courtis]

change_root allows/requires moving the following:

• find-file

toggle and open will still require core.get_explorer

[alex-courtis]

find-file cannot move as it operates without an explorer instance