Add a 2D molecule / SMILES viewer

[Luthaf]

There are a lot of ML applications working with topological description of molecules (only atoms & bonds between them), an it would be nice to allow them to use chemiscope as well.

The standard format for these application is SMILES strings, which describe a subset of atoms from which the molecule can be reconstructed, usually without the H. For example, "CCO" is ethanol. We could use this nice SMILES parser & renderer in javascript: https://github.com/reymond-group/smilesDrawer.

One possible limitation is that atoms might be missing from the SMILES string, and thus there is no way to consistently map from one atom to a single point in the map. From this, it would make sense to only allow SMILES when in "structure" mode.