[SyllableBasedPhonemizer API] Make alias extension restriction to same subbank optional

[lottev1991]

This PR makes it so that aliases can now also be extended across different subbanks in syllable-based phonemizers. Note that the option to restrict it to only the same subbank has not been removed; I've simply made it so that it will be up to the phonemizer developers to decide.

The reason I am making this PR is because the current spec can cause issues when extending diphthongs in certain languages (including English) across different pitches and/or voice colors.

In order to re-implement the previous spec into a phonemizer, the code should look something like this:

if (CanMakeAliasExtension(syllable) && AreTonesFromTheSameSubbank(syllable.tone, syllable.vowelTone))

or, in negative form:

if (!CanMakeAliasExtension(syllable) || !AreTonesFromTheSameSubbank(syllable.tone, syllable.vowelTone))