[css-text] Some comments on the categorization of scripts

[xfq]

https://drafts.csswg.org/css-text-3/#script-groups

clustered scripts

Clustered scripts have discrete units and break only at word boundaries, but do not use visible word separators. They prioritize stretching spaces, but comfortably admit inter-character spacing for justification. The clustered scripts include, but are not limited to, the following Unicode scripts: Khmer, Lao, Myanmar, New Tai Lue, Tai Le, Tai Tham, Tai Viet, Thai

Unlike many other Southern Brahmic scripts, Tai Viet do uses spaces between words (since 1980s).

(There are also scripts that do not exactly match the definition above, but do not use spaces as word separators, like Balinese, Batak, Javanese, and Tibetan.)

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cursive scripts

Cursive scripts do not admit gaps between their letters for either justification or letter-spacing. The following Unicode scripts are included: Arabic, Mandaic, Mongolian, N’Ko, Phags Pa, Syriac

Adlam and Hanifi Rohingya are also cursive scripts.

[fantasai]

Added Hanifi Rohingya to the cursive list. It looks like Adlam has a non-cursive form, however, so it doesn't belong in the list. https://en.wikipedia.org/wiki/Adlam_script

Wrt Tai Viet, how do its justification practices compare to other SE Asian scripts? That's primarily what this appendix is for.