Change get_range/1 to allow returning nil and add syntax corpus

[zachallaun]

Closes #106
Related to #105

While I was writing the proposal in #105, I suggested has_range? thinking that it might be possible to always be able to calculate a range for a 3-tuple. When adding the syntax corpus, however, I found that that doesn't seem to be the case for some injected nodes in interpolations or for empty blocks ({:__block__, [], []}).

So I went ahead and attempted the get_range refactor and it's not bad at all.

Note that this is a breaking change currently. If we don't want to break the contract of Sourceror.get_range/1, we could easily do something like this:

@spec get_range(Macro.t(), keyword()) :: range
def get_range(quoted, opts \\ []) do
  Sourceror.Range.get_range(quoted, opts) ||
    raise ArgumentError, "cannot get range for: #{inspect(quoted)}"
end

@spec fetch_range(Macro.t(), keyword()) :: {:ok, range} | :error
def fetch_range(quoted, opts \\ []) do
  case Sourceror.Range.get_range(quoted, opts) do
    %{} = range -> {:ok, range}
    nil -> :error
  end
end

[zachallaun]

Looking at CI, I need to pull out some additional syntax to only run on later Elixir versions. I'll fix & rebase in just a little bit!

[zachallaun]

Phew, finally got CI passing 😅

[doorgan]

Awesome work! 💜