[CSS] text updates, Fx 70

[chrisdavidmills]

Trello: https://trello.com/c/FdS8hdQo/97-css-text-eg-text-text-decoration-fx-70

Acceptance criteria

1. • Rename text-decoration-width to text-decoration-thickness, to match compat & current spec
2. • Enable "text-decoration-skip-ink" by default on Nightly/early-beta
3. • pref text-underline-offset on in Nightly/early-beta
4. • pref text-decoration-thickness on by default in Nightly/early-beta
5. • [css-text-decor-4] Implement text-decoration-skip-ink
6. • [css-text-decor-4] Let text-decoration-skip-ink, text-decoration-thickness, and text-underline-offset ride the trains to release (Fx 71 milestone, but also fixed for Fx70).

[chrisdavidmills]

This work is now done to the best of my knowledge. I have:

• Updated and tidied up https://developer.mozilla.org/en-US/docs/Web/CSS/text-decoration-thickness
• Updated and tidied up https://developer.mozilla.org/en-US/docs/Web/CSS/text-underline-offset
• Updated the property data for these properties, adding in formal syntax and other information for the above two properties, and updating the text-decoration data so that it includes text-decoration-thickness in the shorthand. See adding syntax data for text-decoration-thickness and text-underline-offset data#387
• Added/updated browser compat data for the aforementioned properties. See updating Fx70 text property support information browser-compat-data#4825
• Added interactive examples for text-decoration-thickness and text-underline-offset (see adding examples for new css text properties - text-decoration-thickne… interactive-examples#1440), although I don't think I'll sgow them until Chrome supports these properties.
• Showed the relevant interactive example on https://developer.mozilla.org/en-US/docs/Web/CSS/text-decoration-skip-ink now it is supported across Firefox and Chromium, as well as generally tidying up the page.
• Updated https://developer.mozilla.org/en-US/docs/Web/CSS/text-decoration to say that text-decoration-thickness is included in the shorthand in supporting browsers. My compat data additions also reflect this (only in Fx 70 for now)
• Mentioned all of this on the release notes: https://developer.mozilla.org/en-US/docs/Mozilla/Firefox/Releases/70#CSS