API migrations for Elm 0.19

[Aaron VonderHaar]

This is a tentative list from the limited info about Elm 0.19

• 🛑 remove elm install ordering workaround (blocked on Elm)
• install test dependencies (blocked on elm-test)
• add elm-lang/browser if necessary
• for packages, move elm files to src/ (or warn)
• move tests to tests/
• update readme
• for packages, warning if license is not in the whitelist
• if more than 5 exposed modules, give warning (see upgrade docs)

js stuff

• package renames -> #33

elm-format stuff

• .program -> Browser.*
• Json.parse returns Json.Error
• name shadowing is not allowed
• (maybe?) case on numbers becomes if/else?