Multi Instance: Refactor: nvim-tree.explorer.node

[alex-courtis]

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

nvim-tree.explorer.node is a stateful singleton. Make it an instantiable class owned by Explorer.

All usages must go through the TreeExplorer singleton.

2024 08 08: explorer.node has nothing specific to explorer; it appears to be a general node class. Close in favour of #2886