OpenXML: Support Excel cell exclusion by style
Suggested by Jonathan [here](https://groups.google.com/g/okapi-users/c/mSh3xBcw3ZY):
> 4. The ability to exclude Excel cells by cell style.
In short, the same option that already exists in the Word section (+ also allow users to manually set a custom style name).
Apparently this is more common than I realized, since our people have seen it as well. So bringing style-based exclusion into XLSX to bring it in line with Word would be useful.
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