Add Check Support for Java 21 Pattern Matching for Switch Syntax: IllegalToken

[mahfouz72]

child of #14961

I have read check documentation: https://checkstyle.org/checks/coding/illegaltoken.html
I have downloaded the latest checkstyle from: https://checkstyle.org/cmdline.html#Download_and_Run
I have executed the cli and showed it below, as cli describes the problem better than 1,000 words

PS D:\CS\test> javac src/Test.java                                               
PS D:\CS\test> cat config.xml                                                                                     
<?xml version="1.0"?>
<!DOCTYPE module PUBLIC
          "-//Checkstyle//DTD Checkstyle Configuration 1.3//EN"
          "https://checkstyle.org/dtds/configuration_1_3.dtd">

<module name="Checker">
  <module name="TreeWalker">
    <module name="IllegalToken">
       <property name="tokens" value="LITERAL_WHEN"/>
    </module>
  </module>
</module>
PS D:\CS\test> cat src/Test.java                                                 
public class Test {
    void test(Object o) {
        switch (o) {
            case Integer i when i == 5 -> {} // violation
            default -> {}
        }
    }
}
PS D:\CS\test> java  -jar checkstyle-10.17.0-all.jar -c config.xml  src/Test.java
Starting audit...
[ERROR] D:\CS\test\src\Test.java:4:28: Using 'when' is not allowed. [IllegalToken]
Audit done.
Checkstyle ends with 1 errors.

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Describe what you expect in detail.

I expected the check to work for any token and it works correctly. no update is needed for this check

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[nrmancuso]

Yep, looks good, we can close this