phoneme: recognise the est, hlg and lng length phonemes.

src/libespeak-ng/compiledata.c

@@ -290,7 +290,7 @@ static keywtab_t keywords[] = {
 	{ "nolink",       tPHONEME_FLAG, phNOLINK },
 	{ "trill",        tPHONEME_FLAG, phTRILL }, // TODO (deprecated): use 'trl' instead
 	{ "palatal",      tPHONEME_FLAG, phPALATAL }, // TODO (deprecated): use 'pzd' instead
-	{ "long",         tPHONEME_FLAG, phLONG },
+	{ "long",         tPHONEME_FLAG, phLONG }, // TODO (deprecated): use 'lng' instead
 	{ "dontlist",     tPHONEME_FLAG, phDONTLIST },
 	{ "brkafter",     tPHONEME_FLAG, phBRKAFTER },
 	{ "rhotic",       tPHONEME_FLAG, phRHOTIC }, // TODO (deprecated): use 'rzd' instead

src/libespeak-ng/phoneme.c

@@ -216,6 +216,14 @@ phoneme_add_feature(PHONEME_TAB *phoneme,
 	case lns:
 		// Not supported by eSpeak.
 		break;
+	// length
+	case est:
+	case hlg:
+		// Not supported by eSpeak.
+		break;
+	case lng:
+		phoneme->phflags |= phLONG;
+		break;
 	// invalid phoneme feature
 	default:
 		return ENS_UNKNOWN_PHONEME_FEATURE;

src/libespeak-ng/phoneme.h

@@ -113,6 +113,10 @@ typedef enum {
 	// fortis and lenis
 	fts = FEATURE_T('f', 't', 's'),
 	lns = FEATURE_T('l', 'n', 's'),
+	// length
+	est = FEATURE_T('e', 's', 't'),
+	hlg = FEATURE_T('h', 'l', 'g'),
+	lng = FEATURE_T('l', 'n', 'g'),
 #	undef FEATURE_T
 } phoneme_feature_t;