Allow settings for filter groups in `PagefindUI`

[nhoizey]

I already opened multiple issues for improvements of filter groups in PagefindUI:

• 567

• 568

• 594

Here's an attempt at setting these features in the PagefindUI initialization:

const search_ui = new PagefindUI({
  filterGroups: {
    "group 1": {
      order: 2,
      sort: "alphanum",
      logic: "OR"
    },
    "group 2": {
      order: 1,
      sort: "results",
      logic: "AND"
    }
  }
});

filterGroups takes an object with an element per filter group where we want to define settings different from the default ones:

setting	default	values	description
order	undefined	number	Sets the order of the group in the list of filter groups. Groups without this setting are listed after the ones with it, in the same alphanumerical order as currently.
sort	alphanum	alphanum, results	Sets the order of filter values in this group. results sorts the filter values by decreasing number of results with this value.
logic	AND	AND, OR	Sets the logic between values from the same group.