The RFC 9460 presentation syntax allows escaping the ',' list separator
in a `SvcParamValue` by writing `\,`. This commit updates `parse_list`
to handle this case.
Once this is done, one of the RFC 9460 test vectors using this feature
can be added to the unit tests. We don't yet support the more
complicated escaped comma using an escaped backslash for delimiter
escape so one remaining test vector for this is left out.
0b70253e86