Comment
Refactor has_valid_accent()
Remove 'uo' from STRIPPED_TERMINAL_VOWELS

It may clash with 'ươ' and prevent typing 'thương'.
Use list to specify SoundTuple's arguments
Comment
Disable ng, nh validation

The ng and nh rules are not really phonetic but spelling rules.
Including them may hinder typing freedom and may prevent typing
unique local names.
Remvove 'ua' from STRIPPED_TERMINAL_VOWELS

It may clash with 'uâ' and prevent typing 'luật'.
Clarify docstring for is_valid_sound_tuple()
Disable tests for ng, nh validation
test_engine.py: Test with skip-non-vietnamese=True by default