Blocks: introduce new package for block management (#17100)

See #17099

packages/blocks/.gitattributes

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+# Files not needed to be distributed in the package.
+.gitattributes      export-ignore
+phpunit.xml.dist    export-ignore
+tests/              export-ignore

packages/blocks/.gitignore

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+vendor
+wordpress
+composer.lock

packages/blocks/README.md

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+# Jetpack Blocks
+
+Register and manage blocks within a plugin. Used to manage block registration, enqueues, and more.
+
+## Usage
+
+## Development
+
+Running tests requires working `svn`, `composer` and `yarn` commands.
+
+Once these are installed, you install the composer dependencies by running:
+
+```bash
+$ composer install --ignore-platform-reqs
+```
+
+### Run PHP unit tests
+
+On a local development environment run:
+```bash
+$ composer phpunit
+```

packages/blocks/composer.json

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+{
+	"name": "automattic/jetpack-blocks",
+	"description": "Register and manage blocks within a plugin. Used to manage block registration, enqueues, and more.",
+	"type": "library",
+	"license": "GPL-2.0-or-later",
+	"require": {},
+	"require-dev": {
+		"phpunit/phpunit": "^5.7 || ^6.5 || ^7.5",
+		"automattic/wordbless": "dev-master"
+	},
+	"autoload": {
+		"classmap": [
+			"src/"
+		]
+	},
+	"scripts": {
+		"phpunit": [
+			"@composer install",
+			"./vendor/phpunit/phpunit/phpunit --colors=always"
+		],
+		"post-update-cmd": "php -r \"copy('vendor/automattic/wordbless/src/dbless-wpdb.php', 'wordpress/wp-content/db.php');\""
+	},
+	"repositories": [
+		{
+			"type": "path",
+			"url": "../*"
+		}
+	],
+	"minimum-stability": "dev",
+	"prefer-stable": true
+}

packages/blocks/phpunit.xml.dist

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+<phpunit bootstrap="tests/php/bootstrap.php" backupGlobals="false" colors="true">
+	<testsuites>
+		<testsuite name="main">
+			<directory prefix="test" suffix=".php">tests/php</directory>
+		</testsuite>
+	</testsuites>
+</phpunit>

packages/blocks/src/class-blocks.php

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+<?php
+/** Blocks package.
+ *
+ * @since 9.0.0
+ *
+ * This package lifts elements from Jetpack's Jetpack_Gutenberg class.
+ * It is now an standalone package reusable outside Jetpack.
+ *
+ * @package automattic/jetpack-blocks
+ */
+
+namespace Automattic\Jetpack;
+
+/**
+ * Register and manage blocks within a plugin. Used to manage block registration, enqueues, and more.
+ *
+ * @since 9.0.0
+ */
+class Blocks {
+
+}
+
+