Editor: Add missing blocks origin to theme.json.

This changeset updates the blocks origin name from core to blocks and adds it to the list of valid origins for `theme.json`.
(See the original fix in [https://github.com//pull/3319 Gutenberg's PR 44363]).

Why?

- This new origin was missing from the list.
- The `core` name is not reflective of what it does, as this data origin is related to block styles, whether they come with WordPress or third-party blocks.
- The existing filter for this piece of data is called `theme_json_blocks`, to reflect it filters "block" data.
- Though `core` origin was used in the past for `default`, this commit reverts it. Why? It was confusing. The goal is to use names that communicate what part of the pipeline are processing (`default > blocks > theme > custom`).

How?

- Renames the string, from `core` to `blocks`.
- Adds `blocks` to the list of valid origins.
- Verifies that the `$theme_json->get_stylesheet()` call uses the proper `$origins` at all times.

Follow-up to [54162], [54251].

Props oandregal, czapla, jorgefilipecosta, scruffian, bernhard-reiter hellofromTonya.
See #56467.


git-svn-id: https://develop.svn.wordpress.org/trunk@54408 602fd350-edb4-49c9-b593-d223f7449a82

src/wp-includes/class-wp-theme-json.php

@@ -50,10 +50,12 @@ class WP_Theme_JSON {
 	 * The sources of data this object can represent.
 	 *
 	 * @since 5.8.0
+	 * @since 6.1.0 Added 'blocks'.
 	 * @var string[]
 	 */
 	const VALID_ORIGINS = array(
 		'default',
+		'blocks',
 		'theme',
 		'custom',
 	);