Blocks: allow enabling Experimental blocks via a filter (#14908)

Follow-up from #14104.

In some environments (like on WordPress.com), it may be easier to use a filter to change the way we load blocks.

class.jetpack-gutenberg.php

@@ -313,6 +313,17 @@ public static function init() {
 			Constants::set_constant( 'JETPACK_BETA_BLOCKS', true );
 		}
 
+		/**
+		 * Alternative to `JETPACK_EXPERIMENTAL_BLOCKS`, set to `true` to load Experimental Blocks.
+		 *
+		 * @since 8.4.0
+		 *
+		 * @param boolean
+		 */
+		if ( apply_filters( 'jetpack_load_experimental_blocks', false ) ) {
+			Constants::set_constant( 'JETPACK_EXPERIMENTAL_BLOCKS', true );
+		}
+
 		/**
 		 * Filter the whitelist of block editor extensions that are available through Jetpack.
 		 *