自动去除VTT class

例如：
```
<c.background-color_transparent.font-family_monospace.font-style_normal.font-weight_normal>[bell tolls]</c>
<c.background-color_transparent.font-family_monospace.font-style_normal.font-weight_normal><b>[bell tolls]</b></c>
<c.background-color_transparent.font-family_monospace.font-style_normal.font-weight_normal>Is it open now,
dough boy?</c>
```

src/N_m3u8DL-RE.Common/Entity/WebVttSub.cs

@@ -15,6 +15,8 @@ public partial class WebVttSub
         private static partial Regex TSValueRegex();
         [GeneratedRegex("\\s")]
         private static partial Regex SplitRegex();
+        [GeneratedRegex("<c\\..*?>([\\s\\S]*?)<\\/c>")]
+        private static partial Regex VttClassRegex();
 
         public List<SubCue> Cues { get; set; } = new List<SubCue>();
         public long MpegtsTimestamp { get; set; } = 0L;
@@ -88,7 +90,7 @@ public static WebVttSub Parse(string text, long BaseTimestamp = 0L)
                         {
                             StartTime = startTime,
                             EndTime = endTime,
-                            Payload = string.Join("", payload.Where(c => c != 8203)), //Remove Zero Width Space!
+                            Payload = RemoveClassTag(string.Join("", payload.Where(c => c != 8203))), //Remove Zero Width Space!
                             Settings = style
                         });
                         payloads.Clear();
@@ -120,6 +122,15 @@ public static WebVttSub Parse(string text, long BaseTimestamp = 0L)
             return webSub;
         }
 
+        private static string RemoveClassTag(string text)
+        {
+            if (VttClassRegex().IsMatch(text))
+            {
+                return VttClassRegex().Match(text).Groups[1].Value;
+            }
+            else return text;
+        }
+
         /// <summary>
         /// 从另一个字幕中获取所有Cue，并加载此字幕中，且自动修正偏移
         /// </summary>