Theme JSON schema: add heading/button key to color definition

[t-hamano]

Follow-up on #49131
Similar to #55675

What?

This PR adds heading/button key to color definition in theme.json schema.

Why?

To allow developers to design theme.json with reference to the correct schema.

Testing Instructions

Create the following JSON file locally, specifying the JSON schema changed by this PR.

{
	"$schema": "https://raw.githubusercontent.com/WordPress/gutenberg/schema/theme-button-heading/schemas/json/theme.json",
	"version": 2
}

• You should see the button and heading keys listed in the settings.color property.
• You should see the button and heading keys listed in the settings.block.{blockName}.color property.
• The default value of true should be set when the button or heading key is selected.

[carolinan]

I find this confusing because it can easily be mixed up with styles > elements > headings.

[t-hamano]

Indeed. What about "Allow users to set heading colors in a block."?

[ajlende]

I'm fine with either phrasing for the description. If we do add "in a block" to the heading description, we should also update the description for button, link, and text too.

[t-hamano]

If we do add "in a block" to the heading description, we should also update the description for button, link, and text too.

Makes sense. The theme.json schema's settings.color.{text|link|heading|button} all have the phrase "in a block". I would like to add similar phrases to these properties in block.json and then merge this PR.