Closes #17

This upgrades shorter_maps to support Elixir 1.16 with the following two changes:

• The [line: line] pattern match is appears to be unreliable for the 2nd field of the tuple in sigil_m/2. This field sometimes comes back empty (which I would expect a failure, and sometimes includes column, which results in a false failure. I updated this to use a Keyword.get/3 rather than the match. If there's a better approach here, I'd appreciate the feedback.
• I also updated the tests and removed the reliance on espec. It's hard for me to gleam the historic value of this library here, but to me it seems like ExUnit is complete enough to give nearly all of the desired behaviors (except doubly nested contexts). I opted to remove espec because it has yet to be updated to Elixir 1.16, and I think relying on just ExUnit will be more sustainable going forward.