[css-text-4] text-spacing replacing spaces

[fantasai]

We had at several points (most recently here and here and here) discussed having the space-inserting values of text-spacing replace U+0020 when it's used in place of the appropriate spacing type.

Specific cases that came up:

• replacing U+0020 when it's used instead of non-breaking thin spaces inside French punctuation
• replacing U+0020 when it's used for ideograph-alpha or ideograph-numeric spacing

This issue is to discuss that proposal: whether to make the default, or opt-in via some mechanism.

[frivoal]

@fantasai, I think this is ready to close, as addressed by the commit above. Agreed?