Allow escaping of characters in the delimiter

Currently the delimiter must be a single character.
Maybe the escaping of characters should be allowed to have quotes as separator.
`@export Test :- dsv{delimiter="\""}.`

This would also allow an alternative formulation for tsv files with `@export Test :- dsv{delimiter="\t"}.` , that can be beneficial when translating Souffle into Nemo, and vice versa.

No file or test is named. Start by locating the dsv export delimiter parsing and trace how quoted and escaped values are handled. Done means delimiters such as an escaped quote and \t are accepted in the forms shown in issue #560.

Written by the indexing model from the issue text.