Add Quenya ⟨iu⟩, ⟨ui⟩ diphthongs

Both are copied from the Finnish phonemes, since Finnish was a major
inspiration for Quenya. This means that the ⟨iu⟩ diphthong is a
“falling” one – according to Appendix E of The Lord of the Rings, this
is the original pronunciation, but by the Third Age (the time in which
The Lord of the Rings is set) it had become a “rising” one, so I may
change the phoneme later, not sure.

phsource/ph_quenya

@@ -0,0 +1,13 @@
+phoneme ui
+  vwl starttype #u endtype #i
+  length 270
+  lng
+  FMT(vdiph/ui_4)
+endphoneme
+
+phoneme iu
+  vwl starttype #i endtype #u
+  length 280
+  lng
+  FMT(vdiph2/iu_2)
+endphoneme

phsource/phonemes

@@ -1921,8 +1921,9 @@ phonemetable py eo
 include ph_pyash
 
 phonemetable qya la
+include ph_quenya
 
-phonemetable sjn qya
+phonemetable sjn la
 
 phonemetable es-la es
 include ph_spanish_la