First drop for data builders

[martinbonnin]

Simple use case is in tests:

    val data = GetStuffQuery.Data {
      nullableInt = null
      nonNullableInt = 42

      this["alias"] = 50
    }

    assertEquals(null, data.nullableInt)
    assertEquals(42, data.nonNullableInt)
    assertEquals(50, data.alias)

Still needed:

• custom scalars support
• java codegen
• default values
• strict mode
• use in our own tests
• (maybe) cleanup the IrType2 stuff

See #4016

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[BoD]

Looks good! 🚀