Add function generation

Description

This MR introduces breaking changes to the I18n workflow.

CLI arguments

You need to define a language as argument to the elm-i18n cli:

elm-i18n generate en_us
elm-i18n watch en_us

Named Holes

{
    "Hello": "Hello {user}!"
}

Function generation

Now there the translations are not longer custom type variants, instead functions with records are arguments are generated. Lets take a look a an exmaple:

Assume this json:

{
    "Hello": "Hello {user}!"
}

will generate:

hello : { user : String } -> String
hello argument = 
    "Hello " ++ argument.user ++ "!"

Now the useage of the I18n module needs to be updated:

-- from this
I18n.t (TidHello "Peter")

-- into this
I18n.hello { user = "Peter" }

Due the dead-code-elimination of the elm compiler, all functions which are not used are removed. This will reduce the bundle a bit.

CI/CD

We will run the test for the generation and a test for the examples in a ci pipeline.