Intent to ship: text-underline-position

[Jonathan Kew]

I'm shortly intending to turn on support for the CSS property
text-underline-position on all platforms.

This feature was developed behind the preference
"layout.css.text-underline-position.enabled".

Other UAs shipping this feature:
* Internet Explorer v6 (an earlier non-standard version of the property)
* MS Edge v12 (an earlier non-standard version of the property)
* Blink/Chrome ('under' value since v33, 'left' and 'right' since v71)
* Opera ('under' value since v20, 'left' and 'right' since v58)

Bug to enable by default:
https://bugzilla.mozilla.org/show_bug.cgi?id=1606997

This property has been enabled by default in Nightly 73 since a month
ago, as of bug 770780. I hope to uplift bug 1606997's patch to beta73 so
that this will ship in Firefox 73.

This feature was previously discussed in the intent-to-prototype
thread:
https://groups.google.com/forum/#!topic/mozilla.dev.platform/zKsl5XCB8LM