feat: calculate metric for percent of text missing (#1701)
### Summary
Missing text is a particularly important metric of quality for the
Unstructured library because information from the document is not being
captured and therefore not usable by downstream applications.
Add function to calculate the percent of text missing relative to the
source transcription. Function takes 2 text strings (output and source)
as input, and returns the percentage of text missing as a decimal.
### Technical Details
- The 2 input strings are both assumed to already contain clean and
concatenated text (CCT)
- Implementation compares the bags of words (frequency counts for each
word present in the text) of each input text
- Duplicated/extra text is not penalized
- Value is limited to the range [0, 1]
### Test
- Several edge cases are covered in the test function (missing text,
duplicated text, spaced out words, etc).
- Can test other cases or text inputs by calling the function with 2 CCT
strings as "output" and "source"
