phoneme_add_feature: support the lat and sib phonemes.

src/libespeak-ng/compiledata.c

 	{ "wavef",        tPHONEME_FLAG, phWAVE },
 	{ "unstressed",   tPHONEME_FLAG, phUNSTRESSED },
 	{ "fortis",       tPHONEME_FLAG, phFORTIS },
-	{ "sibilant",     tPHONEME_FLAG, phSIBILANT },
+	{ "sibilant",     tPHONEME_FLAG, phSIBILANT }, // TODO (deprecated): use 'sib' instead
 	{ "nolink",       tPHONEME_FLAG, phNOLINK },
 	{ "trill",        tPHONEME_FLAG, phTRILL }, // TODO (deprecated): use 'trl' instead
 	{ "vowel2",       tPHONEME_FLAG, phVOWEL2 },

src/libespeak-ng/phoneme.c

 	ejc,
 	imp,
 	vwl,
+	lat,
+	sib,
 };
 
 static MNEM_TAB features[] = {
 	{ "ejc", ejc },
 	{ "imp", imp },
 	{ "vwl", vwl },
+	{ "lat", lat },
+	{ "sib", sib },
 	// invalid phoneme feature
 	{ NULL,  inv },
 };
 	case clk:
 	case ejc:
 	case imp:
+	case lat:
 		// Not supported by eSpeak.
 		break;
 	case vwl:
 		phoneme->type = phVOWEL;
 		break;
+	case sib:
+		phoneme->phflags |= phSIBILANT;
+		break;
 	// invalid phoneme feature
 	default:
 		return create_name_error_context(context, ENS_UNKNOWN_PHONEME_FEATURE, feature);