Update CSS text-spacing property spec to align to the latest draft

[MurakamiShinyu]

Recently, the CSS text-spacing property (CSS Text Level 4) draft spec has made the following changes:

• Add ideograph-alpha and ideograph-numeric to normal (default)
• Do not apply ideograph-alpha/ideograph-numeric spacing to non-zero margin/border/padding
• Use 1/8 of full-width spacing for ideograph-alpha/ideograph-numeric

We should update our CSS text-spacing implementation to align to this latest spec.

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The related csswg-drafts issue (already resolved) about ideograph-alpha/ideograph-numeric spacing:

• [css-text-4] Make ideograph-alpha and ideograph-numeric part of text-spacing: normal w3c/csswg-drafts#6950

Also, there have been the following issues, already resolved or about to be resolved.

• [css-text-4] text-spacing: trim-end is better than allow-end for the normal value w3c/csswg-drafts#7055
• [css-text-4] Propose 'text-spacing: space-first' (trim-start-except-first-line) as a normal behavior w3c/csswg-drafts#2462

Our CSS text-spacing implementation already incorporates these proposals, i.e., the trim-end and space-first are part of text-spacing: normal. So the normal behavior becomes equivalent to space-first trim-end trim-adjacent ideograph-alpha ideograph-numeric.

We should also incorporate the last suggestion in the comment by @frivoal:

• Modify hanging-punctuation: first to hang a paragraph-initial ideographic space as well.

[MurakamiShinyu]

Implemented in #1084.

Tested in #1084 (comment):

Tests: (source: https://github.com/vivliostyle/vivliostyle.js/tree/master/packages/core/test/files/text-spacing)

• text-spacing-ja.html
  • Test with Vivliostyle Viewer: https://vivliostyle.vercel.app/#src=https://raw.githack.com/vivliostyle/vivliostyle.js/master/packages/core/test/files/text-spacing/text-spacing-ja.html
• hanging-punctuation-first-indent-ja.html
  • Test with Vivliostyle Viewer: https://vivliostyle.vercel.app/#src=https://raw.githack.com/vivliostyle/vivliostyle.js/master/packages/core/test/files/text-spacing/hanging-punctuation-first-indent-ja.html