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

[t-hamano]

Follow-up on #49131
Similar to #55674

What?

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

Why?

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

How?

Testing Instructions

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

{
	"$schema": "https://schemas.wp.org/trunk/block.json",
	"apiVersion": 3,
	"name": "test/test",
	"title": "Test"
}

• You should see the button and heading keys listed in the supports.color property.
• The default value of true false should be set when the button or heading key is selected.

[github-actions]

Flaky tests detected in 6db2eb1.
Some tests passed with failed attempts. The failures may not be related to this commit but are still reported for visibility. See the documentation for more information.

🔍 Workflow run URL: https://github.com/WordPress/gutenberg/actions/runs/6674433953
📝 Reported issues:

• [Flaky Test] should partially select with shift + click (@webkit) #53524 in /test/e2e/specs/editor/various/multi-block-selection.spec.js

[carolinan]

I tested it manually, following the instructions.

The default value of true should be set when the button or heading key is selected.

I believe you meant that the default value should be false? VSCode shows the default as false, I believe the updated schema is correct.

Screenshot from VSCode:

[t-hamano]

I believe you meant that the default value should be false

Yes, that's right 😅 I also updated Testing Instructions.

[t-hamano]

Thanks for the review, @carolinan!