include linkcheckmd as source in any Diagnostic

Fixes #60

Previously a user had no idea that the diagnostics reported
by this extension in the Problems tab (or in a hover state)
where from linkcheckmd. This resolves that issue.

extension.ts

@@ -241,6 +241,7 @@ function createDiagnostic(severity: DiagnosticSeverity, markdownLink, lineText:
     // Create the diagnostic object
     let diag = new Diagnostic(range, message, severity);
     diag.code = code;
+    diag.source = "linkcheckmd";
     // Return the diagnostic
     return diag;
 }