Print unnecessary escape characters

Follow up of twigphp#4176, twigphp#4123 and twigphp#2712.

Now that we have deprecated unnecessary escape in v3, we can change the behavior in v4.

With this change, unnecessary will no longer be ignored, but handled like any other character.

This allows for writing fully qualified class names like this:
```twig
{{ constant('App\Entity\User::SOME_CONSTANT') }}
```

Instead of having to escape the `\` in v3:
```twig
{{ constant('App\\Entity\\User::SOME_CONSTANT') }}
```

src/Lexer.php

@@ -428,12 +428,7 @@ private function stripcslashes(string $str, string $quoteType): string
 
             if (isset(self::SPECIAL_CHARS[$nextChar])) {
                 $result .= self::SPECIAL_CHARS[$nextChar];
-            } elseif ($nextChar === '\\') {
-                $result .= $nextChar;
-            } elseif ($nextChar === "'" || $nextChar === '"') {
-                if ($nextChar !== $quoteType) {
-                    trigger_deprecation('twig/twig', '3.12', 'Character "%s" at position %d does not need to be escaped anymore.', $nextChar, $i + 1);
-                }
+            } elseif ($nextChar === '\\' || $nextChar === $quoteType) {
                 $result .= $nextChar;
             } elseif ($nextChar === '#' && $i + 1 < $length && $str[$i + 1] === '{') {
                 $result .= '#{';
@@ -451,8 +446,7 @@ private function stripcslashes(string $str, string $quoteType): string
                 }
                 $result .= chr(octdec($octal));
             } else {
-                trigger_deprecation('twig/twig', '3.12', sprintf('Character "%s" at position %d does not need to be escaped anymore.', $nextChar, $i + 1));
-                $result .= $nextChar;
+                $result .= '\\' . $nextChar;
             }
 
             $i++;

tests/LexerTest.php

@@ -12,7 +12,6 @@
  */
 
 use PHPUnit\Framework\TestCase;
-use Symfony\Bridge\PhpUnit\ExpectDeprecationTrait;
 use Twig\Environment;
 use Twig\Error\SyntaxError;
 use Twig\Lexer;
@@ -22,8 +21,6 @@
 
 class LexerTest extends TestCase
 {
-    use ExpectDeprecationTrait;
-
     public function testNameLabelForTag()
     {
         $template = '{% § %}';
@@ -195,6 +192,18 @@ public function testStringWithEscapedDelimiter(string $template, string $expecte
 
     public function getStringWithEscapedDelimiter()
     {
+        yield '{{ \'App\Test\' }} => App\Test' => [
+            '{{ \'App\\Test\' }}',
+            'App\Test',
+        ];
+        yield '{{ "foo \\\' bar" }} => foo \' bar' => [
+            '{{ "foo \\\' bar" }}',
+            "foo \' bar",
+        ];
+        yield '{{ \'foo \" bar\' }} => foo \" bar' => [
+            '{{ \'foo \\" bar\' }}',
+            'foo \" bar',
+        ];
         yield '{{ \'\x6\' }} => \x6' => [
             '{{ \'\x6\' }}',
             "\x6",
@@ -229,43 +238,6 @@ public function getStringWithEscapedDelimiter()
         ];
     }
 
-    /**
-     * @group legacy
-     * @dataProvider getStringWithEscapedDelimiterProducingDeprecation
-     */
-    public function testStringWithEscapedDelimiterProducingDeprecation(string $template, string $expected, string $expectedDeprecation)
-    {
-        $this->expectDeprecation($expectedDeprecation);
-
-        $lexer = new Lexer(new Environment(new ArrayLoader()));
-        $stream = $lexer->tokenize(new Source($template, 'index'));
-        $stream->expect(Token::VAR_START_TYPE);
-        $stream->expect(Token::STRING_TYPE, $expected);
-
-        // add a dummy assertion here to satisfy PHPUnit, the only thing we want to test is that the code above
-        // can be executed without throwing any exceptions
-        $this->addToAssertionCount(1);
-    }
-
-    public function getStringWithEscapedDelimiterProducingDeprecation()
-    {
-        yield '{{ \'App\Test\' }} => AppTest' => [
-            '{{ \'App\\Test\' }}',
-            'AppTest',
-            "Since twig/twig 3.12: Character \"T\" at position 5 does not need to be escaped anymore.",
-        ];
-        yield '{{ "foo \\\' bar" }} => foo \' bar' => [
-            '{{ "foo \\\' bar" }}',
-            'foo \' bar',
-            "Since twig/twig 3.12: Character \"'\" at position 6 does not need to be escaped anymore.",
-        ];
-        yield '{{ \'foo \" bar\' }} => foo " bar' => [
-            '{{ \'foo \\" bar\' }}',
-            'foo " bar',
-            'Since twig/twig 3.12: Character """ at position 6 does not need to be escaped anymore.',
-        ];
-    }
-
     public function testStringWithInterpolation()
     {
         $template = 'foo {{ "bar #{ baz + 1 }" }}';