[Iguana] Move ConditionStack to conditionstack.h

Summary:
To build ScriptInterpreter, we need to have access to ConditionStack in interpreter.h.

The simplest way is moving this into its own header file. The class is so simple that it doesn't need a corresponding cpp file.

Test Plan: ninja check

Reviewers: Fabien, #bitcoin_abc

Reviewed By: Fabien, #bitcoin_abc

Differential Revision: https://reviews.bitcoinabc.org/D16766