Update CSS text-spacing-trim property to support the latest spec change

[MurakamiShinyu]

CSS WG decided to change the text-spacing-trim property value spec. (w3c/csswg-drafts#9511 (comment))

The CSS Working Group just discussed [css-text][text-spacing] Visual regressions of line-start at portals and news sites, and agreed to the following:

• RESOLVED: Accept Murakami's proposal to simplify to text-spacing: normal | trim-start | space-first | trim-auto | space-all | trim-all | auto
• RESOLVED: remap property value space for text-spacing

The accepted proposal (w3c/csswg-drafts#9511 (comment)):

I agree with the normal as the initial value which implies space-start trim-adjacent allow-end.

Other necessary text-spacing-trim values will be:

• trim-start implies trim-start trim-adjacent allow-end
• space-first implies space-first trim-adjacent allow-end
• trim-auto implies trim-start trim-adjacent trim-end
• space-all implies space-start space-adjacent space-end
• trim-all (trim anywhere)

So the value syntax will be:

normal | trim-start | space-first | trim-auto | space-all | trim-all

Other value keywords (space-start, allow-end, etc.) and combinations are probably not necessary.

[MurakamiShinyu]

Vivliostyleの現在の実装から変更になる点

• デフォルトが space-first から normal (= space-start trim-adjacent allow-end) に変更。つまり：
  • 折り返し行頭約物は詰めない（以前は詰めるだった）
  • 行末約物は行に収まらないときだけ詰める（以前は常に詰めるだった）
• 行頭側（space-first または trim-first）だけを指定したとき、行末側は allow-end の動作（行に収まらないときだけ詰める）

以前からサポートしていたキーワードとその組み合わせもサポートし続けることが可能。