[css-text-3] Visible Control Characters Shipping Coordination

[fantasai]

In #7249 we agreed to open a new issue to handle coordination for shipping the “make control characters visible” behavior specced in CSS Text L3, so opening this issue to track.

Where we left things back in May:

• Blink just passes through to the current font, so whether the control code is visible depends on the font used to render it.
• Gecko has implemented visible control chars behind a flag, which is on in Nightly builds.

Mozilla's position was:

emilio: I don't recall seeing issues with this in Nightly/Beta and we've had it for a long time
jfkthame: Also think we should ship it. Do occasionally see characters show up in Nightly that are invisible elsewhere, but it's rare.
jfkthame: And in all cases the users are better served by sites fixing their broken data

Tab asked @chrishtr to coordinate for Blink. smfr was unsure about WebKit's situation. See minutes.