Intent to prototype: CSS property `hyphenate-character`

[Jonathan Kew]

Summary: This property allows authors to control what character (or
string) should be displayed when a line of text is hyphenated by the
browser during layout. (Not all languages/writing systems use the same
"hyphen" character.)

Bug: https://bugzilla.mozilla.org/show_bug.cgi?id=1746187

Standard: https://www.w3.org/TR/css-text-4/#hyphenate-character

Platform coverage: all

Preference: layout.css.hyphenate-character.enabled

DevTools bug: n/a

Other browsers: Webkit and Blink have both been shipping this feature as
a prefixed property (-webkit-hyphenate-character). Raised
https://github.com/w3c/csswg-drafts/issues/6887 to determine whether
this is stable enough to ship (unprefixed).

web-platform-tests: Tests under /css/text/parsing and /css/text/hyphens
are added as part of bug 1746187.