Big footnote updates

[ddsjoberg]

What changes are proposed in this pull request?

• Updates to the handling of footnotes. Previously, header footnotes were handled with modify_footnote() and modify_table_styling(footnote). It was possible to also include footnotes in the table body with modify_table_styling(footnote), but this was largely a hidden feature. Also confusingly, a special abbreviation footnote was handled with modify_footnote(abbreviation=TRUE).

  In this update, we now export separate user-facing functions for each of these with clearer names and scope: modify_footnote_header(), modify_footnote_body(), and modify_abbreviation(). As the names indicate, the modify_footnote_header() and modify_footnote_body() functions place footnotes in the header and table body. Abbreviations are now treated like source notes and do not have footnote markers associated with them. We also export functions remove_footnote_header(), remove_footnote_body(), and remove_abbreviation() to remove previously assigned footnotes and abbreviations.

  Also, multiple footnotes may now reference the same cell in the table or column header by utilizing the modify_footnote_header(replace=FALSE), modify_footnote_body(replace=FALSE) argument.

• Previously, source notes were an undocumented feature and only a single source note could be included in a table. We now export modify_source_note() and remove_source_note() to add and remove any number of source notes. Also, when merging and stacking tables, previously due to the one source note limit, only the first source note was retained. Now all source notes will be included below the resulting table. This is different behavior compared to previous versions of the package and in rare cases may result in a different source note. Moreover, kableExtra output now supports source notes, where previously they were omitted.

If there is an GitHub issue associated with this pull request, please provide link.
closes #1675

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Reviewer Checklist (if item does not apply, mark is as complete)

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[ayogasekaram]

this looks amazing! Thanks @ddsjoberg

[edelarua]

I love it!! Less overlapping/cluttered and much easier to understand what each function is meant to do. Very nice that you can add new footnotes without replacing the old ones :)