[FEATURE] Preprocessor should check validity of parameters

Created by: denispelli

This is all spelled out in the EasyEyes Threshold manual. The preprocessor should check validity of: parameter names parameters types Any request fonts should be checked against the list of fonts in the scientist's Font folder in Pavlovia, and the short list of commonly available fonts (like Arial and Verdana). Also, a current limitation of PsychoJS is that all conditions in a block must have the same number of trials. The preprocessor should enforce this.