Changes of version 15.0.0

• [Breaking] Metric override name change: xheight → xHeight.
• Add variant selector for dot and comma diacritics (#1309).
• Fix enclosed glyphs' placement when having glyphs with different Y-shifting (#1310).
• Add metric override for archDepth and smallArchDepth (#1313).
• Fix feature application on OBJECT REPLACEMENT CHARACTER (U+FFFC) and REPLACEMENT CHARACTER (U+FFFD) (#1314).