Adding JSON I/O for molecules

[isty2e]

Sometimes it is convenient to exchange molecules in JSON format rather than SMILES or .sdf files. Fortunately, rdChem.Mol <-> JSON is already implemented in rdkit.Chem.rdMolInterchange.MolsToJSON and rdkit.Chem.rdMolInterchange.JSONToMols. This can be implemented in either datamol.io or datamol.mol, though I'm not sure exactly where, and what the signatures should be: For example, implementing this in datamol.mol.to_mol() with JSON string input will require another if-else, which will slow down the method slightly. Perhaps datamol.mol.to_mol(mol: str | Mol | dict) might be better, but the JSON can contain multiple molecules so it's not a good interface in the first place.