delta: refactor into standalone module

[LucasWagler]

Description

WIP

Refactor delta into its own standalone module, with a new enableGitIntegration option.

Many submodules under the Git module deserve to be broken out into their own spaces. Delta is a great example, as it can be used without Git.

Checklist

• Change is backwards compatible.

• Code formatted with nix fmt or
  nix-shell -p treefmt nixfmt-rfc-style keep-sorted --run treefmt.

• Code tested through nix-shell --pure tests -A run.all
  or nix build --reference-lock-file flake.lock ./tests#test-all using Flakes.

• Test cases updated/added. See example.

• Commit messages are formatted like

  {component}: {description}
  
  {long description}
  

  See CONTRIBUTING for more information and recent commit messages for examples.

• If this PR adds a new module

  • Added myself as module maintainer. See example.

Maintainer CC

@khaneliman
@rycee

[LucasWagler]

Running TODO list:

• Move & polish tests
• Ensure new test files are wired up & asserted
• Remove any commented pieces & TODOs
• Decide where to direct the old enable option to (enable vs. enableGitIntegration)
• What should the new enable and enableGitIntegration default to? Tie them together?
• Should we move/remove the assertion for ensuring only a single diff tool is used?
• Ensure config file is created regardless of Git being enabled? (It uses .gitconfig as its config file)

[ambroisie]

I agree that delta (and other pagers) should have its (their) own module. Good first step

I do have a nit to review for the module options.

[ambroisie]

IMO this should default to false for enable and true for enableGitIntegration.

Suggested change

	enable = mkEnableOption "" // {
	default = cfg.enableGitIntegration;
	description = ''
	Whether to enable the {command}`delta` syntax highlighter.
	See <https://github.com/dandavison/delta>.
	'';
	};
	enableGitIntegration = mkEnableOption "" // {
	description = ''
	Whether to enable Git integration for the {command}`delta` syntax highlighter.
	See <https://github.com/dandavison/delta>.
	'';
	enable = mkEnableOption "" // {
	description = ''
	Whether to enable the {command}`delta` syntax highlighter.
	See <https://github.com/dandavison/delta>.
	'';
	};
	enableGitIntegration = mkEnableOption "" // {
	default = true;
	description = ''
	Whether to enable Git integration for the {command}`delta` syntax highlighter.
	See <https://github.com/dandavison/delta>.
	'';

[LucasWagler]

Your suggestion seems reasonable to me, at the expense of new users needing to explicitly disable Git integration when adding delta to their config. That should be fine, as I'd expect a majority of users to want Git integration enabled, anyway. I'll look at implementing it!

Ideally, programs.git.delta.enable would be split-renamed into both programs.delta.enable and programs.delta.enableGitIntegration. We could then keep the default as false for both new options.

[ambroisie]

Related to my previous comment, this would need to use a custom message with mkRemovedOptionModule or mkChangedOptionModule to point at both programs.delta.{enable,enableGitIntegration}.

[ambroisie]

Makes me wish that dandavison/delta#1959 had been better received.