[cling,core] Improve .x arg parsing (ROOT-10097):

Dropbox folders (among others) can contain parentheses.
Without this patch, ROOT and cling misinterpret those directories as arguments.
Instead, first find the end of the ".x" line.
If the previous token was a closing paren, we assume that the preceding
tokens (up to the non-nested opening paren) belong to the argument.