[css-text-4] timing of the virtual word boundary insertion and of the U+200B conversion

https://drafts.csswg.org/css-text-4/#word-boundary-detection and https://drafts.csswg.org/css-text-4/#word-boundary-expansion say that the virtual word boundary insertion and U+200B expansion happen "before CSS Text Module Level 3 §white-space-phase-1".

This is so that the (possibly virtual) U+200B converted into a space can participate into white space collapsing. This is good, but phase 1 also invokes Segment Break Transformation Rules, and having these be affected by word-boundary-detection and word-boundary-expansion seem undesirable.

it would mean that

aaaa​
bbb

would be rendered as

aaa​bbb

but that if word-boundary-expansion were set to ideographic-space, it would then turn into

aaa　 bbb

rather than the more desirable

aaa　bbb

So, both of these opperations should be changed to happen

after Segment Break Transformation Rules, but before the rest of Phase I: Collapsing and Transformation.

cc: @fantasai