Basic zsh completion support

- This adds basic zsh support similarly to
  existing bash completion
- New command "completion zsh"
- Fix internal recursive command completion model for
  cases with deep nested commands
- Fixes #927

spring-shell-standard-commands/src/main/java/org/springframework/shell/standard/commands/Completion.java

@@ -1,5 +1,5 @@
 /*
- * Copyright 2022 the original author or authors.
+ * Copyright 2022-2023 the original author or authors.
  *
  * Licensed under the Apache License, Version 2.0 (the "License");
  * you may not use this file except in compliance with the License.
@@ -20,6 +20,7 @@ import org.springframework.shell.standard.AbstractShellComponent;
 import org.springframework.shell.standard.ShellComponent;
 import org.springframework.shell.standard.ShellMethod;
 import org.springframework.shell.standard.completion.BashCompletions;
+import org.springframework.shell.standard.completion.ZshCompletions;
 
 /**
  * Command to create a shell completion files, i.e. for {@code bash}.
@@ -52,4 +53,10 @@ public class Completion extends AbstractShellComponent {
 		BashCompletions bashCompletions = new BashCompletions(resourceLoader, getCommandCatalog());
 		return bashCompletions.generate(rootCommand);
 	}
+
+	@ShellMethod(key = "completion zsh", value = "Generate zsh completion script")
+	public String zsh() {
+		ZshCompletions zshCompletions = new ZshCompletions(resourceLoader, getCommandCatalog());
+		return zshCompletions.generate(rootCommand);
+	}
 }