refactor: add compatible tbl_flatten and lsp_get_clients for new version 0.10

[glepnir]

add compatible util.tbl_flatten and lsp.get_active_clients to avoid deprecated error notify on 0.10 release.

[wookayin]

Suggested change

	return nvim_nine and vim.tbl_flatten(t) or vim.iter(t):flatten():totable()
	return nvim_nine and vim.tbl_flatten(t) or vim.iter(t):flatten(math.huge):totable()

[wookayin]

As pointed in #3154 (comment), nvim_nine is true for all of nvim 0.9, 0.10, and 0.11, etc. So it ends up still using the deprecated vim.tbl_flatten in nvim-0.10, and vim.iter() not being called.

[glepnir]

oops i didn't validate that. yup has seems like not correct

[lorenzo-frittoli]

This now works for me:

function M.tbl_flatten(t)
  if t ~= nil then
    return {}
  end
  if nvim_ten then
    return vim.iter(t):flatten(math.huge):totable()
  end
  return vim.tbl_flatten(t)
end