When using the CLI and a property is deprecated we emit a warning by using the `x-deprecated` field. However API consumers have no way to know that a property is deprecated at the moment.
This converts `x-deprecated` to `@deprecated` comments
The tests verify that the output of the rule is the same as the input, and will
error if not (with a call to action). The binary produces the golden output.
We have to check the result of the transformation into git because internally
we cannot synchronize quicktype into google3.