Add Sindarin ⟨ae⟩, ⟨ai⟩, ⟨oe⟩, ⟨ui⟩ diphthongs

For ⟨ae⟩, we used to use the Latin [[aI]] phoneme, which sounds like
/ae/. Call that phoneme [[aE]] for us and introduce a separate [[aI]]
one, which sounds more like /ai/ (since Sindarin has both, and they’re
supposed to sound different, though Appendix E of The Lord of the Rings
notes that there is nothing closely corresponding to ⟨ae⟩ in English and
that it may be pronounced like ⟨ai⟩). Furthermore, for ⟨oe⟩, just remove
the TODO – the Latin phoneme is called [[OI]] but sounds more like /oe/
than /oi/, so it’s actually just fine for our purposes. Finally, the
⟨ui⟩ diphthong is copied from Finnish, just like in Quenya.

phsource/ph_sindarin

@@ -0,0 +1,18 @@
+phoneme aE
+  vwl starttype #a endtype #e
+  length 270
+  FMT(vdiph/ae)
+endphoneme
+
+phoneme aI
+  vwl starttype #a endtype #i
+  length 240
+  FMT(vdiph/ai)
+endphoneme
+
+phoneme ui
+  vwl starttype #u endtype #i
+  length 270
+  lng
+  FMT(vdiph/ui_4)
+endphoneme

phsource/phonemes

@@ -1924,6 +1924,7 @@ phonemetable qya la
 include ph_quenya
 
 phonemetable sjn la
+include ph_sindarin
 
 phonemetable es-la es
 include ph_spanish_la