One rule to rule them all

[jfmengels]

Hi @jxxcarlson @mateusfpleite

This PR makes it so that every single transformation is not a rule in itself but can be combined into a rule. This makes it possible - or at least easier - to run only a single transformation, without needing to create a whole new project. That makes it easier to run rules from a template.

elm-review --template --fix-all author/package/lamdera --rules AddAuth

(and you can still create multiple templates if you'd like to, both become possible).

It is absolutely fine for you to dismiss this PR/idea, if you think this is a bad idea. I had a bit of fun doing it anyway.

This includes a few bits of cleanup I did prior to the larger API changes. Some are already of note, like stopping the exposing of CustomError which was unnecessary, and that is technically a breaking change. I enabled a few elm-review rules that should help identify issues related to docs and things not being exposed when they should (that wouldn't have caught CustomError though).

---

The API and documentation very likely need a bit of polish. I may have missed updating some documentation, but they APIs could probably be made nicer to use. It's likely a bad idea to publish a new version right after this gets merged.

A few ideas (for which I'd like feedback):

• Rename Installation to something else that makes more sense. I chose it to get going, but I'm not attached to the name.
• Name every configuration function config. By the end of the change, I named every function config (Initializer.config, ...) but for the first ones I gave them different names, such as ElementToList.add. I think we could name them all config for consistency, although that would make AddImport.qualified look out of place.

---

Another idea, which could probably be a bit simpler, is to group installations per module, rather than passing the module name to every single installation. That could look something like this:

config = List Rule
config =
    [ Install.rule "AddPage"
        [ ( "ViewMain"
          , [ Import.qualified [ "Pages." ++ pageTitle ]
              |> Install.Rule.addImport
            -- , ...
            ]
          )
        , ( "Route"
          , [ ElementToList.add "routesAndNames"
                [ "(" ++ pageTitle ++ "Route, \"" ++ routeName ++ "\")" ]
                |> Install.Rule.addElementToList
            -- ...
            ]
          )
        ]
    ]

[mateusfpleite]

Thanks a lot for this PR, @jfmengels! The changes have made the API much better and simpler, especially in cases where many rules are needed. I made a few adjustments just to ensure consistency in the documentation.

If @jxxcarlson agrees, I'm happy to merge it.