Implement is_complete_request

[SylvainCorlay]

The is_complete request is implemented in xeus-cling at https://github.com/jupyter-xeus/xeus-cling/blob/0.15.0/src/xinterpreter.cpp#L258-L282.

This is required for meaningful usage of xeus-cpp in a Jupyter console.

For this purpose, we would need a counterpart to cling's input validator.

[vgvassilev]

Let's cc @sunho who is actively on the related feature in clang-repl: D143142, D143144 and D143148.