Post Navigation Link: add design controls (color, text decoration, and font family)

[jffng]

What?

Adds experimental controls for link color, text decoration, and font family to the Post Navigation Link block.

Why?

It would be useful to control these style attributes via the editor. Text decoration is maybe questionable, but I think the link color and font family are additions whose use cases should be evident.

How?

Updates the theme's block.json

Testing Instructions

1. Activate a theme whose "settings" > "appearanceTools" > true, for example emptytheme.
2. Go to the site editor, add a post navigation link.
3. Verify you can set its link color, font family, and text decoration:

Screenshots or screencast

cc @WordPress/block-themers

[draganescu]

While color is obvious the others are not that clear. Will all fonts we have in that dropdown provide those arrows? As you said, the decoration is even more questionable, what would an all caps strike through "next" symbol do? I think let's add just the color and open an issue about the text options. Maybe we can move away from text in the block?

[jffng]

Will all fonts we have in that dropdown provide those arrows

No I don't think so, which is why being able to choose the font family I think is useful.

I agree text decoration is unnecessary, will remove it from this PR.

[draganescu]

😁 thanks for the update @jffng

Excuse my persistence, but I am still unclear what font family is for because:

• hopefully one day soon we’ll use some SVG for the symbols (like hopefully we won’t depend on text for the separator dotted style as well)
• like you said as well, not all fonts support our symbols so we’re creating a way for users to make bad looking results. And this one is quite exotic, as getting some ª instead of the chevrons is baffling.

[scruffian]

Yeah I think font family is potentially confusing here. Shall we reduce it to just color?

[jffng]

If the link has a text label, it would be useful to control the font family. For example in a theme I'm working on, the post nav links are the heading font, not the body font:

Current	Intended

I don't think it's a big deal to set this in the theme's CSS especially if you can get weird characters for the chevron, but wanted to clarify why I thought it could be useful.

[richtabor]

If the link has a text label, it would be useful to control the font family.

I agree.

[scruffian]

Makes sense to me, let's merge.

[jffng]

e2e admin tests failing, tried to rebase but it didn't help.