Fixed string representation in option files (json in particular)
repr is now used used when creating option files from a given Gaudi configuration so that escape sequences are properly handled. The previous implementation did use str and escaped '"' by hand. But r strings for example were not properly supported properly so far resulting in warning messages when loading back with python 3.12
Note that this MR was made possible thanks to the work made on the StringGrammar in !1839 (merged).
A Test has been added, with a previously problematic case (an r string containing ".").