Automatically expand abbreviations

[1ec5]

Though other Vietnamese IMEs have built-in text substitution facilities, it’d be out of scope for this extension, since there are already Firefox extensions that serve this purpose well. Instead, I think it’d be more useful to automatically expand abbreviations common in handwriting, for example:

• ddc. → “đc̣” → “được”
• ng` → “ng̀” → “người”

This feature would complement anything we do for #74.