Add support of record format 'D' - ASCII text

## Background
The new option 'record_format' supports F, V, VB formats.
The format 'D' is supported by using 'is_text' + 'is_record_sequence'. This is not very convenient.

More on data formats: https://www.ibm.com/docs/en/zos/2.3.0?topic=files-selecting-record-formats-non-vsam-data-sets

## Feature
Add the direct support for format 'D' - ASCII text

