Export the encoder and decoder of `Variables`

[choonkeat]

Closes #12

1. converts queryOperation.inputs: QueryObjectInput to QueryCompositeNonFragmentOutput
2. then we can getCompositeDecoder for our queryOperation.inputs
3. update generateExports and generateEncodersAndDecoders to include our new decoders

having a new decoder to work with QueryCompositeNonFragmentOutput | QueryObjectInput seem more correct but got hairy fast, so i went with the input conversion approach

[choonkeat]

  running elm make
    ✖  Error: Command failed: elm make src/Main.elm --output=generated/index.html at step (/home/travis/build/harmboschloo/graphql-to-elm/tests/gen/test-browser/index.ts:44:23)
        'Error: Command failed: elm make src/Main.elm --output=generated/index.html'undefined

ah found out why this failed. i'll check that later

[harmboschloo]

  running elm make
    ✖  Error: Command failed: elm make src/Main.elm --output=generated/index.html at step (/home/travis/build/harmboschloo/graphql-to-elm/tests/gen/test-browser/index.ts:44:23)
        'Error: Command failed: elm make src/Main.elm --output=generated/index.html'undefined

ah found out why this failed. i'll check that later

if you run elm make src/Main.elm --output=generated/index.html in tests\gen\test-browser then you'll get the proper elm error. For the tests I disabled the elm output because it messed up the test output.

[choonkeat]

avoided duplicating the encoders & decoders in 2a012b4 🙇

[harmboschloo]

Was this one finished? I kind of missed it 😅 .

[choonkeat]

yes it's done! 👏 . i've been using my branch for a while

[harmboschloo]

Ah cool, so no need to rush then 😄 I'll see when I've got time to have a proper look.

having a new decoder to work with QueryCompositeNonFragmentOutput | QueryObjectInput seem more correct but got hairy fast, so i went with the input conversion approach

Yeah, I'd like to have another look at that.

Also I'm thinking of having this optional, and not have it enabled by default.