Add two APIs for getting stylistic set labels

* hb_ot_layout_feature_get_characters
* hb_ot_layout_feature_get_name_ids

However HarfBuzz currently doesn't expose an API for retrieving the actual
information associated with NameId from the `name` table and that should be
done separately.

docs/harfbuzz-sections.txt

diff --git a/docs/harfbuzz-sections.txt b/docs/harfbuzz-sections.txt
index 9472af4..fccfcb0 100644
--- a/docs/harfbuzz-sections.txt
+++ b/docs/harfbuzz-sections.txt
@@ -479,7 +479,9 @@
 HB_OT_TAG_JSTF
 hb_ot_layout_collect_lookups
 hb_ot_layout_collect_features
+hb_ot_layout_feature_get_characters
 hb_ot_layout_feature_get_lookups
+hb_ot_layout_feature_get_name_ids
 hb_ot_layout_feature_with_variations_get_lookups
 hb_ot_layout_get_attach_points
 hb_ot_layout_get_glyph_class