Add 'อ' rule to royin.py

[wannaphong]

What does this changes

• Add 'อ' rule

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[coveralls]

Coverage increased (+0.003%) to 95.797% when pulling f6314da on update-royin into 68c3e81 on dev.

[bact]

After refactoring in #566, the rule of a syllable starts with "อ" may need to adjust.

[veer66]

@bact I don't know whether I commit to the right place. #568 🤦‍