ISTF validator

At some point, we should start with validator, which will validate the ISTF array for correctness.

### Use cases

- It will be needed to control the output from CSS parser or from a objects->istf transformer.
- It will ease the life of those who try to implement their own *-to-istf compiler and want to be sure they are doing it right.
- One could use it to validate a package installed from npm with istf inside and make sure the produced istf is correct.

### Tasks

- Make sure validator knows all constraints we put into ISTF #44
- Make sure it outputs meaningful error messages

Start by reading the ISTF constraints referenced in issue #44 and the use cases in this issue. Define the validator's scope around checking ISTF arrays and reporting meaningful errors; done means all listed constraints are covered and invalid input produces actionable messages.

Written by the indexing model from the issue text.