Expose Gecode scheduling constraints with (start, duration, end) tasks to MiniZinc

In Gecode, many [scheduling constraints](https://www.gecode.org/doc-latest/reference/group__TaskModelIntScheduling.html) constraints take a triple of `start`, `duration`, and `end` variables for each task, while the coresponding constraints in MiniZinc work with only a `start` and `duration`.

The idea is to add new custom constraints in [gecode.mzn](https://github.com/Gecode/gecode/blob/develop/gecode/flatzinc/mznlib/gecode.mzn) for the `cumulative`, `cumulatives`, and `unary` variants and matching posting functions in [the registry](https://github.com/Gecode/gecode/blob/develop/gecode/flatzinc/registry.cpp).