JiraLinkInlineParser.cs - prevent Jira key's to start with a digit

src/Markdig/Extensions/JiraLinks/JiraLinkInlineParser.cs

@@ -40,7 +40,13 @@ public override bool Match(InlineProcessor processor, ref StringSlice slice)
  var startKey = slice.Start;
  var endKey = slice.Start;
 
- //read as many uppercase characters as required - project key
+ // the first character of the key can not be a digit.
+ if (current.IsDigit())
+ {
+ return false;
+ }
+
+ // read as many uppercase characters or digits as required - project key
  while (current.IsAlphaUpper() || current.IsDigit())
  {
  endKey = slice.Start;