Read more block: add support for text color

[MaggieCabrera]

What?

This PR adds support for text color on the read more block

Why?

When this block was introduced in #37649 it was mentioned that the way to control the color would be:

because is just a link, the way to control the color of the link is through the highlight control in RichText toolbar options.

That brings a couple of problems:

• a theme can't define a default color for all instances of the block via theme.json
• using the highlight tool creates an extra <mark> tag that is not very semantic if we just want to change colors for stylistic reasons only

How?

I just added support for text color on block.json

Testing Instructions

On a query block with the read more block in it, try to change the text color.
It should also work if we add this to theme.json:

"core/read-more": {
    "color": {
        "text": "red"
    }
}

Screenshots or screencast

[scruffian]

LGTM

[ntsekouras]

Looks good, thanks!

FYI I hadn't added text color in the initial PR because it felt a bit inconsistent settng color to a link from text color. The reasons you mention though are good ones, especially the control from theme.json.