Add combining left arrow to stretchy data for STIX fonts

Currently the stretchy right arrow (U+2192) has the combining right-arrow above (U+20D7) in its list of single-character versions (so that vector over-arrows will be produced properly). The stretchy left arrow should include the combining left-arrow above (U+20D6) for those fonts that include it (namely the STIX fonts) in order to be consistent.