vi: move Northern Vietnamese voice setting to a new variant in !v

espeak-ng-data/lang/aav/vi

 
 words 1
 pitch 90 170
-
-formant 1 87 82 76
-formant 2 94 56 42
-formant 3 93 52 130
-formant 4 110 76 65
-formant 5  102 45 20
-formant 6  40 50 50
-formant 7 60 50 60
-formant 8 100 50 40
-
-tone 900 225  1600 200 2400 150

espeak-ng-data/voices/!v/Nguyen

+language variant
+name Nguyen
+
+pitch  90 170
+
+formant 1 87 82 76
+formant 2 94 56 42
+formant 3 93 52 130
+formant 6  40 50 50
+formant 7 60 50 60
+formant 8 100 50 40
+
+tone 900 225  1600 200 2400 150