Revamp the project and add support for credo diff

[Wigny]

After having worked on a similar solution that would allow me to generate a CodeClimate report for the mix credo diff command, I took the liberty of replicating these changes to this project so hopefully the community can also benefity from it.

While making the changes I noticed that the project is using old Elixir and deps versions, so I decided to bump them also, revamping the project.

Some of the changes included in this MR are:

• increased the minimal supported Elixir version to 1.15 (as it is the minimum version still officially maintened);
• made the Jason dependency optional as the JSON module is available now since Elixir v1.18;
• updated the deps;
• added support for generating the CodeClimate report when executing credo diff;
• added tests and a CI workflow;

Please let me know if you desire me to split this MR in two smaller ones.

[hauleth]

Looks awesome. Just small little thing to handle.

[hauleth]

I would make it more extensive check, to check that if JSON is not available, then Jason has to be available, otherwise it should fail.

Suggested change

	@json_lib if Code.ensure_loaded?(JSON), do: JSON, else: Jason
	@json_lib case do
	Code.ensure_loaded?(JSON) -> JSON
	Code.ensure_loaded?(Jason) -> Jason
	true -> raise "No JSON encoder found, if you are using Elixir <= 1.17 then add `:jason` to your dependencies"
	end

Or something like that.

[Wigny]

Good idea, but instead of that, I just updated the dependencies list to always depend on :jason if running on an Elixir version lower than 1.18.

That should ensure Jason is always used on older Elixir versions, and that we rely on JSON on newer versions when Jason is not installed.

The mix compile --no-optional-deps check on CI should ensure that this config works on all versions.