std processing: add type attribute

[Intelligent2013]

Source: https://github.com/metanorma/mn-samples-bsi/issues/2
If value contains : and 4 digits at end end, then add the attribute type="dated":

<std type="dated">
   <std-ref>ISO Guide 73:2009</std-ref>
</std>

else type="undated":

<std type="undated">
    <std-ref>ISO 19011</std-ref>
</std>

[Intelligent2013]

For mn xml:

<bibitem>PD 6079-4:2006 (Book)</bibitem>

mn2sts sets non-correct value in @type, due (Book) at the end:

<std-ref type="undated">PD 6079-4:2006 (Book)</std-ref>```

Expected result is:

<std-ref type="dated">PD 6079-4:2006 (Book)</std-ref>

Need to improve algorithm.

[Intelligent2013]

Done.