[Zipper] Fix search_pattern/2 and move_to_cursor/2

[zachallaun]

I found a shared issue with search_pattern/2 and move_to_cursor/2: When __cursor__() was found, both functions would stop matching, so you'd get a positive result even if other elements of the pattern do not match:

iex(1)> alias Sourceror.Zipper, as: Z
Sourceror.Zipper

iex(2)> zipper = "[[[:foo], :bar], :baz]" |> Sourceror.parse_string!() |> Z.zip()
#Sourceror.Zipper<
  #root
  {:__block__, [...], [...]}
>

iex(3)> Z.search_pattern(zipper, "[__cursor__(), :qux]")
#Sourceror.Zipper<
  {:__block__, [...],
   [
     [{:__block__, [...], [[{:__block__, [...], [:foo]}]]},
      {:__block__, [...], [:bar]}]
   ]}
  #...
>

iex(4)> Z.move_to_cursor(zipper, "[[__cursor__(), :bar], :qux]")
#Sourceror.Zipper<
  {:__block__, [...],
   [[{:__block__, [...], [:foo]}]]}
  #...
>

This PR fixes that issue and improves the documentation and tests.

[doorgan]

Thanks for the fix and the docs improvements!