Allow addition of footnotes without supplying any locations

[rich-iannone]

We sometimes find that notes in the footnotes section can be without reference to any particular part of the table. Most often, these notes without marks precede those that do contain marks (it would be confusing to mix the marked and unmarked notes together in any other way, especially if they appear in paragraph form).

This PR makes it so that the default locations parameter is NULL and a footnote (still a required argument) will have no reference to a table cell. The order of tab_footnote() calls is still preserved but is split between location-based footnotes and the non-location variety (which always appears first).

Here is an example:

library(gt)

exibble %>%
  gt() %>%
  tab_footnote(footnote = "A footnote.") %>%
  tab_footnote(
    footnote = "A location note. Appears after the first two notes",
    locations = cells_body(1, 1)
  ) %>%
  tab_footnote(footnote = "A second footnote.") %>%
  opt_footnote_marks(marks = letters) %>%
  tab_options(footnotes.multiline = FALSE)

Fixes: #879