Fix plus superscript parsing (#791)

Author: xoofx

src/Markdig.Tests/Specs/EmphasisExtraSpecs.generated.cs

@@ -65,13 +65,13 @@ public void ExtensionsSuperscriptAndSubscript_Example003()
             // The following Markdown:
             //     One quintillionth can be expressed as 10^-18^
             //     
-            //     Daggers^†^ and double-daggers^‡^ can be used to denote notes.
+            //     Daggers^†^ and double-daggers^‡^ can be used to denote notes. Plus signs^+^ and entity plus signs^+^ can be used in superscript and subscript: H~+~ and H~+~.
             //
             // Should be rendered as:
             //     <p>One quintillionth can be expressed as 10<sup>-18</sup></p>
-            //     <p>Daggers<sup>†</sup> and double-daggers<sup>‡</sup> can be used to denote notes.</p>
+            //     <p>Daggers<sup>†</sup> and double-daggers<sup>‡</sup> can be used to denote notes. Plus signs<sup>+</sup> and entity plus signs<sup>+</sup> can be used in superscript and subscript: H<sub>+</sub> and H<sub>+</sub>.</p>
 
-            TestParser.TestSpec("One quintillionth can be expressed as 10^-18^\n\nDaggers^†^ and double-daggers^‡^ can be used to denote notes.", "<p>One quintillionth can be expressed as 10<sup>-18</sup></p>\n<p>Daggers<sup>†</sup> and double-daggers<sup>‡</sup> can be used to denote notes.</p>", "emphasisextras|advanced", context: "Example 3\nSection Extensions / Superscript and Subscript\n");
+            TestParser.TestSpec("One quintillionth can be expressed as 10^-18^\n\nDaggers^†^ and double-daggers^‡^ can be used to denote notes. Plus signs^+^ and entity plus signs^&plus;^ can be used in superscript and subscript: H~+~ and H~&plus;~.", "<p>One quintillionth can be expressed as 10<sup>-18</sup></p>\n<p>Daggers<sup>†</sup> and double-daggers<sup>‡</sup> can be used to denote notes. Plus signs<sup>+</sup> and entity plus signs<sup>+</sup> can be used in superscript and subscript: H<sub>+</sub> and H<sub>+</sub>.</p>", "emphasisextras|advanced", context: "Example 3\nSection Extensions / Superscript and Subscript\n");
         }
     }
 

src/Markdig.Tests/Specs/EmphasisExtraSpecs.md

@@ -27,10 +27,10 @@ Certain punctuation characters are exempted from the rule forbidding them within
 ```````````````````````````````` example
 One quintillionth can be expressed as 10^-18^
 
-Daggers^†^ and double-daggers^‡^ can be used to denote notes.
+Daggers^†^ and double-daggers^‡^ can be used to denote notes. Plus signs^+^ and entity plus signs^+^ can be used in superscript and subscript: H~+~ and H~+~.
 .
 <p>One quintillionth can be expressed as 10<sup>-18</sup></p>
-<p>Daggers<sup>†</sup> and double-daggers<sup>‡</sup> can be used to denote notes.</p>
+<p>Daggers<sup>†</sup> and double-daggers<sup>‡</sup> can be used to denote notes. Plus signs<sup>+</sup> and entity plus signs<sup>+</sup> can be used in superscript and subscript: H<sub>+</sub> and H<sub>+</sub>.</p>
 ````````````````````````````````
 
 ## Inserted

src/Markdig/Helpers/CharHelper.cs

@@ -80,7 +80,7 @@ public static class CharHelper
 
     [MethodImpl(MethodImplOptions.AggressiveInlining)]
     private static bool IsPunctuationException(char c) =>
-        c is '−' or '-' or '†' or '‡';
+        c is '−' or '-' or '+' or '†' or '‡';
 
     [MethodImpl(MethodImplOptions.AggressiveInlining)]
     private static bool IsPunctuationException(Rune c) =>