Minimal code for exporting custom tags

[Aditya Mahajan <adityam@umich.edu>]

Hi,

What is needed to enable export of user defined macros? Consider the following example:


```
\def\startmycommand{\dostarttagged{mycommand}\empty}
\def\stopmycommand {\dostoptagged}

\setupbackend[export=xml, level=0]

\starttext
\startsection[title={Sample document}]
   \startparagraph
     This is a paragraph
   \stopparagraph
   \startmycommand
     This is mycommand
   \stopmycommand
\stopsection
\stoptext
```

If I compile using `context --trackers=structures.tags filename`, I get

```
backend         > tags > U+0031:1 => U+0031:1 : 00004 document>1 section>1 sectioncaption>1 sectionnumber>1
backend         > tags > U+0053:S => U+0074:t : 00005 document>1 section>1 sectioncaption>1 sectiontitle>1
backend         > tags > U+0054:T => U+0068:h : 00007 document>1 section>1 sectioncontent>1 paragraph>1
backend         > tags > U+0054:T => U+0064:d : 00008 document>1 section>1 sectioncontent>1 mycommand>1
```

so the tag is applied. I can also verify this by looking at the PDF which contains

```
<< /K 27 0 R /ParentTree 2 0 R /RoleMap << /document /Div /mycommand /Span /paragraph /P /section /Sect /sectioncaption /Div /sectioncontent /Div /sectionnumber /H /sectiontitle /H >> /Type /StructTreeRoot >>
```

and the tag `mycommand` is used at appropriate location.

However, the exported xml file contains no tag:

```
<?xml version="1.0" encoding="UTF-8" standalone="yes" ?>

<!--

     input filename   : test-tag
     processing date  : 2020-05-18T03:00:33-04:00
     context version  : 2020.04.30 11:10
     exporter version : 0.35

-->

<!-- This export file is used for filtering runtime only! -->

<document context="2020.04.30 11:10" date="2020-05-18T03:00:33-04:00" file="test-tag" language="en" version="0.35" xmlns:m="http://www.w3.org/1998/Math/MathML">
  <metadata>
  </metadata>
  <section detail="section" chain="section" implicit="1" level="3">
   <sectioncaption>
    <sectionnumber>1</sectionnumber>
    <sectiontitle>Sample document</sectiontitle>
   </sectioncaption>
   <sectioncontent>
    <paragraph>This is a paragraph</paragraph>
   </sectioncontent>
  </section>
</document>
```

What am I missing in order to pass user-defined macros to the exporter?

Thanks,
Aditya

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