feat: improved mapping for missing chipper elements (#1431)
This PR updates
[TYPE_TO_TEXT_ELEMENT_MAP](https://github.com/Unstructured-IO/unstructured/blob/bd33a52ee0fcec3928db171e6d717e70521c5aef/unstructured/documents/elements.py#L551)
with additional mapping for `chipper` elements:
```
“Threading”: NarrativeText,
“Form”: NarrativeText,
“FieldName”: Title,
“Value”: NarrativeText,
“Link”: NarrativeText,
```