Allow setup to be run multiple times

[alex-courtis]

Fail use case:

• nvim-tree setup called
  • nvim-web-devicons setup called with no arguments
• nvim-web-devicons setup called with user overrides
  • NOP as setup already called

Success use case:

• nvim-web-devicons setup called with user overrides
  • overrides applied
• nvim-tree setup called
  • NOP as setup already called

Related:
Plugin checks first as per telescope:

https://github.com/nvim-telescope/telescope.nvim/blob/35f94f0ef32d70e3664a703cefbe71bd1456d899/lua/telescope/utils.lua#L542

README known issue

Possible solution:

1. Add a setup argument preserve, to not override configuration, plugins call with that
2. Always override, nvim-tree will break, cannot use