Allow adding footnotes without a location parameter

[JohannesNE]

It would be useful to add footnotes with tab_footnote() without specifying a location.
This should just add an entry to the footnote text without a superscript number.

My specific use case is to explain a symbol used in the table.

tab_source() can nearly do this, but it is displayed in a separate section

[rich-iannone]

This is a great idea! And definitely now is the time to do so (since footnotes and source notes were recently fixed up).

[rich-iannone]

Will create an issue from this. Thanks again.