Proper line breaking for Khmer, Thai, Lao, and Burmese (Myanmar)

[chearon]

These scripts don't separate words with spaces, and require a dictionary. In line breaking, iterate scripts as well as everything else. If the script is one of these, switch to a specialized break iterator.

This library can break all 4, but need to look into efficiency. Should probably use a trie.

Wikipedia says the first 3 are related through the Khmer writing system:

• Khmer script, for the language of Cambodia
• Thai script, for the language of Thailand
• Lao script, for the language of Laos

But Burmese is not:

• Burmese alphabet (Myanmar)

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Sort of related: Vietnam borders Cambodia and Laos, Vietnamese people today use a variant of Latin. I may need to tweak font selection at some point in order to properly select a Vietnamese font for the entire run instead of having a mix of a Latin and Vietnamese font, since I don't think Latin fonts necessarily support all of Vietnamese.