Converts links to the params of the dependency editor and validates them.

iex> link = GanttEx.Link.new!(id: 1, source: 1, target: 2, lag: 2)
iex> GanttEx.LinkForm.params(link)
%{"type" => "finish_to_start", "lag" => "2"}

iex> GanttEx.LinkForm.cast(%{"type" => "start_to_start", "lag" => "-1"})
{:ok, %{type: :start_to_start, lag: -1}}

iex> GanttEx.LinkForm.cast(%{"type" => "start_to_start", "lag" => "soon"})
{:error, [lag: :invalid_number]}