[css-text-3] word-break: keep-all should not suppress break opportunities after Ideographic Spaces

[frivoal]

The spec defines keep-all as:

Breaking is forbidden within “words”: implicit soft wrap opportunities between typographic letter units (or other typographic character units belonging to the NU, AL, AI, or ID Unicode line breaking classes [UAX14]) are suppressed

U+3000 IDEOGRAPHIC SPACE is not included in any of the listed classes, so the break opportunity that is normally found after it should be preserved. However, it is not in Firefox.

While that is merely an implementation bug, it's a relatively easy mistake to make to assume (incorrectly) that IDEOGRAPHIC SPACE belongs to the class ID=IDEOGRAPHIC.

I don't think the normative prose needs any change, but a note (and a test) seem appropriate.

[frivoal]

(cc @fantasai, since we talked about this)

[frivoal]

I made a test case covering U+3000 as well as U+3001, since neither spaces nor punctuation should be affected, but some browsers get it wrong: web-platform-tests/wpt#13211

I also made a pull request for an editorial improvement to the spec to try and make it clearer that U+3000 isn't affected: #3145