Qualified fn module generation

[lue-bird]

• adds a codegen package fn modules generator
• adds all package fn modules for elm/: core, html, parser, random, json. If we in the future want to support elm/svg or elm-explorations/test for example, this will take a manual step of elm-codegen run on the downloaded elm.json
• adds an elm-review rule to replace the manual tuples + imports
• applies the necessary fixes

I've looked through the leftover tuples and none of them were e.g. misspelled fns, so the rule seems to have caught everything.

If you think anything could be improved, like adding doc comments for the declarations and/or module, leave a comment.

[github-actions[bot]]

The branch can be tried out by running:

elm-review --template jfmengels/elm-review-simplify/preview

[jfmengels]

The #237 branch had conflicts so I had to rebase both branches and force push onto your branch. So please pull the changes before you make new commits.