Draft: Add a utility for text transforms

See https://www.w3.org/TR/css-text-3/#text-transform-property for the corresponding CSS functionality.

This is related to Pango's text segmentation functionality, and thus makes sense to provide as part of Pango.

Doing it at this level is not quite right though.

What really needs to happen is that the transformation should happen when we are populating the buffer with Unicode in pango_hb_shape. The slight complication there is that we need to be prepared for transforms such as ß -> SS and translate the cluster indices accordingly.

Also, it should probably be an attribute, not a layout property.