mbrola/de*: support the /3/ and /3:/ vowels

The /3/ vowel is raised to /@/. This matches the way the English
/3:/ is pronounced (i.e. English /3:/ is realized as /@:/).

The de1, de2 and de3 voices support /3/ and not /3:/. This is
because they do not have the diphones for words like /h3:/. In
this case, the /3:/ sounds should be realized as their pre-merged
/IR/, /UR/ and /ER/ forms as found in Scottish English.