Migrate commands definition inside setup()

[antonk52]

With modern package managers disabling built in commands is awkward and undocumented. This PR moves the definition of colorizer commands into the setup function.

Example of disabling builtin commands currently

-- lazy.nvim
require({
  {
    'NvChad/nvim-colorizer.lua',
    init = function()
      vim.g.loaded_colorizer = 1
    end,
    opts = {}
  }
})

-- packer
use {
  'NvChad/nvim-colorizer.lua',
  setup = function()
    require('colorizer').setup()
  end,
  config = function()
    vim.g.loaded_colorizer = 1
  end,
}

With this PR disabling builtin commands can be done as easy as

-- lazy.nvim
require({
  {
    'NvChad/nvim-colorizer.lua',
    opts = {
      default_commands = false
    }
  }
})

-- packer
use {
  'NvChad/nvim-colorizer.lua',
  setup = function()
    require('colorizer').setup({
      default_commands = false
    })
  end,
}