Create an endpoint to verify gval code

[SchawnnDev]

Actually you can save a gval code without checking if it will work or not.

In the editor there is actually no syntax checker, then the simplest way to check it would be to send the code to the engine, the engine evaluate it and respond with true/false and the error.

Maybe there is a syntax checker included in gval that we can use, otherwise we only check for an error to appear

Later: create a front syntax checker for gval