ISO/IEC Coding Guidelines: code and preformatted text

[Intelligent2013]

Source: #190

Currently, mnconvert converts both code and preformat into:

[source]
--
...
---

but they should processed differently for backward Metanorma XML to NISO STS XML conversion. But at this moment I don't figure out how to separate them.

[Intelligent2013]

code Best Attribute Practice (https://www.niso-sts.org/TagLibrary/niso-sts-TL-1-0-html/element/code.html):
The various semantic attributes should be used to name the type of code, the language, the intended platform(s), etc.

preformat Best Practice (https://www.niso-sts.org/TagLibrary/niso-sts-TL-1-0-html/element/preformat.html)
Although it is possible to use <preformat> to tag any format-preserving object, some of these objects have semantic elements that should be used in preference, for example:
Programming code, scripts, pseudo-code, DTDs and Schemas, and XML instances should be tagged with the [<code>](https://www.niso-sts.org/TagLibrary/niso-sts-TL-1-0-html/element/code.html) element, making full use of its attributes to identify the type of code.

So, if sourcecode doesn't have the attribute lang, then it will be converted to the element preformat.

[Intelligent2013]

Done.