Fix deprecation warning for tbl_flatten

[topaxi]

Newer nvim versions have an iter interface to several table/list operations and deprecated tbl_flatten.

This should fix the deprecation warning in a backwards compatible fashion.

[Dmitrev]

@norcalli could you have a look at this? This issue starts showing in the latest nvim nightly.

- WARNING vim.tbl_flatten is deprecated. Feature will be removed in Nvim 0.13
  - ADVICE:
    - use vim.iter(…):flatten():totable() instead.
    - stack traceback:
        /home/user/.local/share/nvim/lazy/nvim-colorizer.lua/lua/colorizer/nvim.lua:96