Completion command for bash

- Add basic support of defining a command `completion bash` which
  outputs a generic bash script which can be used in a user environment.
- Idea for completion is copied from go's cobra library what comes for
  a bash dance itself.
- Goes through command registry, builds a model for command structure
  and uses antlr st4 for templating bash.
- Should give foundation to create other completions just like in cobra.
- Currently as we don't know a root-command in a generic way, option
  `spring.shell.command.completion.root-command` is required user to set.
- Fixes #343

pom.xml

@@ -24,6 +24,7 @@
 	<properties>
 		<jline.version>3.21.0</jline.version>
 		<jcommander.version>1.81</jcommander.version>
+		<antlr-st4.version>4.3.1</antlr-st4.version>
 	</properties>
 
 	<modules>
@@ -98,6 +99,11 @@
 				<version>${jcommander.version}</version>
 				<optional>true</optional>
 			</dependency>
+			<dependency>
+				<groupId>org.antlr</groupId>
+				<artifactId>ST4</artifactId>
+				<version>${antlr-st4.version}</version>
+			</dependency>
 		</dependencies>
 	</dependencyManagement>