Performance: Move config related code from nvim-tree.lua to config.lua

[alex-courtis]

Part of #3231 : Performance: Load Only Necessary Modules On setup and requiring API

Create a new config.lua module containing:

• constants like DEFAULT_OPTS, ACCEPTED_TYPES
• all validation functionality like validate_options
• move any config functionality from M.setup into a function in config.lua that is the single mutation endpoint.
• a getter for the current config with API nvim_tree.api.config.current() that returns a deep clone
• as above for default()

Config is referred to as opts/options, which is confusing as opts is used for optional function parameters. Refer to everything as config. Ensure that help and help-update.sh is updated accordingly.

M.config, local _config and M.setup opts must be removed from nvim-tree.lua. Replace them with references to config.lua getter.