Trailing case adjustment doesn't handle Dutch

[Manishearth]

    let cm = CaseMapper::new();
    let nl = langid!("nl");
    let mut options = TitlecaseOptions::default();
    options.trailing_case = Some(TrailingCase::Lower);
    assert_eq!(
        cm.titlecase_segment_with_only_case_data_to_string("ij", &nl, default_options),
        "IJ"
    );

Instead if IJ, we get Ij. This is because the way we implement TrailingCase::Unchanged is that we switch over to lowercasing once we process the first character:

icu4x/components/casemap/src/internals.rs

Lines 72 to 75 in 78c809c

	if IS_TITLE_CONTEXT {
	if self.titlecase_tail_casing == TrailingCase::Lower {
	mapping = MappingKind::Lower;
	} else {

AIUI, Dutch is the only tailoring that ever cares about more than the first character when titlecasing; so we mostly just need to ensure the trailing case handling can check for this case.