Note self-synchronizing encodings for later

lib/Encoding/GenericEncoding.php

@@ -15,6 +15,7 @@ trait GenericEncoding {
     protected $dirtyEOF = 0;
     protected $errMode = self::MODE_REPLACE;
     protected $allowSurrogates = false;
+    protected $selfSynchronizing = false;
 
     public $posErr = 0;
 

lib/Encoding/SingleByteEncoding.php

@@ -9,6 +9,8 @@ namespace MensBeam\Intl\Encoding;
 abstract class SingleByteEncoding implements StatelessEncoding {
     use GenericEncoding;
 
+    protected $selfSynchronizing = true;
+
     public function nextChar(): string {
         // get the byte at the current position
         $b = @$this->string[$this->posChar];

lib/Encoding/UTF16.php

@@ -9,6 +9,8 @@ namespace MensBeam\Intl\Encoding;
 abstract class UTF16 implements Encoding {
     use GenericEncoding;
 
+    protected $selfSynchronizing = true;
+
     public function nextCode() {
         $lead_b = null;
         $lead_s = null;

lib/Encoding/UTF8.php

@@ -12,6 +12,8 @@ class UTF8 implements StatelessEncoding {
     const NAME = "UTF-8";
     const LABELS = ["unicode-1-1-utf-8", "utf-8", "utf8"];
 
+    protected $selfSynchronizing = true;
+
     public function nextCode() {
         // this function effectively implements https://encoding.spec.whatwg.org/#utf-8-decoder
         // optimization for ASCII characters