fix: respect `<pre>` tag order in `partition_html` (#1197)
### Summary
Closes #1184. Updates `partition_html` to respect the ordering of
`<pre>` tags in HTML documents.
### Testing
The elements in the following example should be in the correct order.
```python
from unstructured.partition.html import partition_html
html_text = """
<pre>The Big Brown Bear</pre>
<div>The big brown bear is growling.</div>
<pre>The big brown bear is sleeping.</pre>
<div>The Big Blue Bear</div>
"""
elements = partition_html(text=html_text)
print("\n\n".join([str(el) for el in elements]))
```
