Allow to create a standard SymbolMap for EBNF

[keilw]

Currently SymbolMap.of(ResourceBundle rb) takes a given ResourceBundle.
We would like to add another static factory method, e.g. SymbolMap.ofEBNF() (similar to the noarg methods in List, Set or Map) or forEBNF() which uses the ResourceBundle for EBNFUnitFormat without exposing its internals, e.g. the bundle path, as that could change in a future version.

So the SymbolMap instance can be enriched via alias() or label() methods before creating an EBNFUnitFormat with it.