Part of https://core.trac.wordpress.org/ticket/53175
Follow-up to #1262

To do

• prepare corresponding Gutenberg PR

How to test

A theme without theme.json (use the TwentyTwentyOne theme):

• Load the front. Look for a global-styles-inline-css embedded stylesheet. The expected result is that none should be found.
• Load the post editor. Search for --wp--preset--color. The expected result is that none should be found.

A theme with theme.json support:

• Load the front. Look for a global-styles-inline-css embedded stylesheet. The expected result is that there's a stylesheet that contains the CSS generated by the engine. Depending on the theme's theme.json, it should be something along these lines:

body{
  --wp--preset--color--black: #000000;
  /* more CSS Custom Properties for the presets */
}
.has-black-color{ color: #000000 !important; }
.has-black-background-color{ background-color: #000000 !important; }
/* other classes for every preset value  */

• Load the post editor. Search for --wp--preset--color. The expected result is that there's an embedded stylesheet that contains the CSS Custom Properties.
• Load the post editor. Search for .has-black-color{ ... }. The expected result is that there's an embedded stylesheet that contains the classes for the presets as well as any other block styles defined by the theme via theme.json.