xmerl: Fix exporting XML comments

[ptome]

Fix issue #5697 by exporting #xmlComment elements when calling export/3 or export_simple/3 and similar functions.

If we want to preserve the original document formatting, this will be impossible to solve only from the export side. The xmerl_scan functions strip white space after comments when creating the element records, and the original text elements are lost.

Top level comments like the example in #5697 will only be exported if we create and export as a document. There is no function to export an #xmlDocument directly, could be a possible enhancement.

The callback module xmerl_xml_indented doesn't indent properly if input data has text elements used for indentation. I allow for indentation of comments like the example in test 45 formatter_pass where there is no indentation in the input. I updated the test case to insert the newly exported comment.

I tried to implement an alternative PR that preserves formatting by changing xmerl_scan to not strip white space after comments. These would be read as extra text elements. Unfortunately this breaks more than 60 unit tests, mostly related to DTD validation. It would be a larger PR and introduce more risk of breaking something. I didn't have time to investigate a solution for this alternative to work.

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[github-actions]

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