[Core] Deduplicate suggest snippets

Multiple undefined steps may generate the same suggested snippet.
Deduplicating these makes copy paste development a little bit easier.

Fixes: #1930

Author: mpkorstanje

core/src/main/java/io/cucumber/core/plugin/DefaultSummaryPrinter.java

@@ -9,12 +9,13 @@
 import io.cucumber.plugin.SummaryPrinter;
 
 import java.io.PrintStream;
-import java.util.ArrayList;
+import java.util.LinkedHashSet;
 import java.util.List;
+import java.util.Set;
 
 public final class DefaultSummaryPrinter implements SummaryPrinter, ColorAware, StrictAware, ConcurrentEventListener {
 
-    private final List<String> snippets = new ArrayList<>();
+    private final Set<String> snippets = new LinkedHashSet<>();
     private final Stats stats = new Stats();
 
     private final PrintStream out;