Blocks: Add support for variations in block.json` file

We integrated variations with block types and the corresponding REST API endpoint in #52688. It's a follow-up patch to add missing support to the `block.json` metadata file when using `register_block_type`.

Some fields for variations are translatable.Therefore, i18n schema was copied over from Gutenberg: https://github.com/WordPress/gutenberg/blob/trunk/packages/blocks/src/api/i18n-block.json. The accompanying implementation was adapted as `translate_settings_using_i18n_schema`.

Props: gwwar, swissspidy, schlessera, jorgefilipecosta.
Fixes #53238.



git-svn-id: https://develop.svn.wordpress.org/trunk@51599 602fd350-edb4-49c9-b593-d223f7449a82

tests/phpunit/data/blocks/notice/block.json

@@ -41,6 +41,14 @@
 			"label": "Other"
 		}
 	],
+	"variations": [
+		{
+			"name": "error",
+			"title": "Error",
+			"description": "Shows error.",
+			"keywords": [ "failure" ]
+		}
+	],
 	"example": {
 		"attributes": {
 			"message": "This is a notice!"