refactor: use getNextCustomAtomLabel from openchemlib

[tpoisseau]

Refs: #62

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[tpoisseau]

As you asked @lpatiny I switch to getNextCustomAtomLabel from openchemlib.

1. getNextCustomAtomLabel don't take care of ] prefix So I have to do some tricks in react-ocl to have same render.
2. https://github.com/cheminfo/openchemlib-js/blob/592f5d3e71ffc312a3117232c76685135de4dc4e/lib/extend/appendGetNextCustomAtomLabel.js#L49 return '1' but should return ]1 as fallback in our context.
3. It does not supports greeks letters. I expect getNextCustomAtomLabel(']α') returns ']β' but it returns '1' (fallback)

[lpatiny]

Works as I expect thanks.