Add support for the stepxmp element

[jhradilek]

DITA 1.3 supports the <stepxmp> element which can be used to provide an example usage of a step. While this element does not allow titles like <example>, <stepxmp> is the logical choice for AsciiDoc example blocks inside of procedure steps.

Note: If this approach works, it should be applied to both task-generated.xsl and task.xsl.

Implementation checklist

• The code changes come with the corresponding test cases
• The code changes pass all tests (run python3 -m unittest in the project directory)
• The code changes come with appropriate documentation