Add SlugRegister so IDs are not duplicated

tests/data/slug_heading.html

@@ -1,10 +1,11 @@
 <h1 id="foo">foo</h1>
 <h1 id="foo-bar">foo bar</h1>
 <h1 id="foobar">foo_bar</h1>
-<h1 id="foobar">foo+bar</h1>
+<h1 id="foobar-1">foo+bar-1</h1>
+<h1 id="foobar-2">foo+bar</h1>
 <h1 id="2rer0ගම්මැද්ද-v-force-ඉනොවේශන්-නේෂන්-සඳහා-එවූ-නි">2rer*(0👍ගම්මැද්ද V FORCE ඉනොවේශන් නේෂන් සඳහා එවූ නි</h1>
-<h2 id="foo">foo</h2>
-<h2 id="foo-bar">foo bar</h2>
-<h2 id="foobar">foo_bar</h2>
-<h2 id="foobar">foo+bar</h2>
-<h2 id="2rer0ගම්මැද්ද-v-force-ඉනොවේශන්-නේෂන්-සඳහා-එවූ-නි">2rer*(0👍ගම්මැද්ද V FORCE ඉනොවේශන් නේෂන් සඳහා එවූ නි</h2>
+<h2 id="foo-1">foo</h2>
+<h2 id="foo-bar-1">foo bar</h2>
+<h2 id="foobar-3">foo_bar</h2>
+<h2 id="foobar-4">foo+bar</h2>
+<h2 id="2rer0ගම්මැද්ද-v-force-ඉනොවේශන්-නේෂන්-සඳහා-එවූ-නි-1">2rer*(0👍ගම්මැද්ද V FORCE ඉනොවේශන් නේෂන් සඳහා එවූ නි</h2>

tests/data/slug_heading.md

@@ -4,6 +4,8 @@
 
 # foo_bar
 
+# foo+bar-1
+
 # foo+bar
 
 # 2rer*(0👍ගම්මැද්ද V FORCE ඉනොවේශන් නේෂන් සඳහා එවූ නි

tests/src/Configurables/HeaderSlugTest.php

@@ -3,6 +3,7 @@
 namespace Erusev\Parsedown\Tests\Configurables;
 
 use Erusev\Parsedown\Configurables\HeaderSlug;
+use Erusev\Parsedown\Configurables\SlugRegister;
 use Erusev\Parsedown\State;
 use PHPUnit\Framework\TestCase;
 
@@ -19,6 +20,7 @@ final class HeaderSlugTest extends TestCase
 
         $this->assertSame(true, $State->get(HeaderSlug::class)->isEnabled());
     }
+
     /**
      * @return void
      * @throws \PHPUnit\Framework\ExpectationFailedException
@@ -32,7 +34,27 @@ final class HeaderSlugTest extends TestCase
 
         $this->assertSame(
             'foo_bar',
-            $HeaderSlug->transform('foo bar')
+            $HeaderSlug->transform(SlugRegister::initial(), 'foo bar')
+        );
+    }
+
+    /**
+     * @return void
+     * @throws \PHPUnit\Framework\ExpectationFailedException
+     * @throws \SebastianBergmann\RecursionContext\InvalidArgumentException
+     */
+    public function testCustomDuplicationCallback()
+    {
+        $HeaderSlug = HeaderSlug::withDuplicationCallback(function (string $t, int $n): string {
+            return $t . '_' . \strval($n-1);
+        });
+
+        $SlugRegister = new SlugRegister;
+        $HeaderSlug->transform($SlugRegister, 'foo bar');
+
+        $this->assertSame(
+            'foo-bar_1',
+            $HeaderSlug->transform($SlugRegister, 'foo bar')
         );
     }
 }