Add 白讀 to gwongzau.js

[iwsfutcmd]

Currently, gwongzau.js only generates 文讀. Gwongzau 白讀 readings are also generally predictable from Middle Chinese readings, and so can be added as an option.

[ayaka14732]

Thank you! But this seems incomplete?

[iwsfutcmd]

Ah, you are right, I apologize! I forgot to push a few changes before I made this PR. I'll do so soon.

[307587]

第276行有少少問題，白讀咁推會推埋落曾攝，但曾攝無咁樣白讀法，咁寫會將曾攝推錯音。 粵語音變合流爲主，建議推白讀音先，再用異讀規則推文讀。