Use wp_is_valid_utf8() and wp_scrub_utf8() from the new utf8.php decoder (#200)

Replaces two instances of the old UTF-8 decoding utilities with the new
utf-8.php toolkit by @dmsnell:

*
https://github.com/WordPress/wordpress-develop/blob/trunk/src/wp-includes/compat-utf8.php
*
https://github.com/WordPress/wordpress-develop/blob/trunk/src/wp-includes/utf8.php

This PR only touches two tactical usages of the old tools:

* Blueprint validation now uses `wp_is_valid_utf8`
* CSSProcessor now uses `wp_scrub_utf8` instead of
`_wp_scrub_utf8_fallback`

More refactoring is coming once there's a faster alternative to
`_wp_scan_utf8`, see
https://core.trac.wordpress.org/ticket/63863#comment:51

Related to #196.

Follows up on #199 and
#197.

## Testing instructions

If the CI passes, we're good. Unicode-related scenarios are covered by
tests.

Author: adamziel

components/Blueprints/class-runner.php

@@ -55,7 +55,7 @@
 use WordPress\HttpClient\Client;
 use WordPress\Zip\ZipFilesystem;
 
-use function WordPress\Encoding\_wp_has_noncharacters_fallback;
+use function WordPress\Encoding\wp_is_valid_utf8;
 use function WordPress\Filesystem\wp_unix_sys_get_temp_dir;
 use function WordPress\Zip\is_zip_file_stream;
 
@@ -379,14 +379,7 @@ private function load_blueprint() {
 		// Validate the Blueprint string we've just loaded.
 
 		// **UTF-8 Encoding:** Assert the Blueprint input is UTF-8 encoded.
-		$is_valid_utf8 = false;
-		if ( function_exists( 'mb_check_encoding' ) ) {
-			$is_valid_utf8 = mb_check_encoding( $blueprint_string, 'UTF-8' );
-		} else {
-			$is_valid_utf8 = ! _wp_has_noncharacters_fallback( $blueprint_string );
-		}
-
-		if ( ! $is_valid_utf8 ) {
+		if ( ! wp_is_valid_utf8( $blueprint_string ) ) {
 			throw new BlueprintExecutionException( 'Blueprint must be encoded as UTF-8.' );
 		}
 

components/DataLiberation/URL/class-cssprocessor.php

@@ -2,10 +2,10 @@
 
 namespace WordPress\DataLiberation\URL;
 
-use function WordPress\Encoding\_wp_scan_utf8;
-use function WordPress\Encoding\_wp_scrub_utf8_fallback;
 use function WordPress\Encoding\utf8_codepoint_at;
 use function WordPress\Encoding\codepoint_to_utf8_bytes;
+use function WordPress\Encoding\compat\_wp_scan_utf8;
+use function WordPress\Encoding\wp_scrub_utf8;
 
 /**
  * Tokenizes CSS according to the CSS Syntax Level 3 specification.
@@ -1528,7 +1528,7 @@ private function consume_ident_start_codepoint( $at ): int {
 	 */
 	private function decode_string_or_url( int $start, int $length ): string {
 		// Fast path: check if any processing is needed.
-		$slice         = _wp_scrub_utf8_fallback( substr( $this->css, $start, $length ) );
+		$slice         = wp_scrub_utf8( substr( $this->css, $start, $length ) );
 		$special_chars = "\\\r\f\x00";
 		if ( false === strpbrk( $slice, $special_chars ) ) {
 			// No special chars - return raw substring (almost zero allocations).