[nus-cs2103-AY2122S1#319] Checkstyle: add ArrayTypeStyle rule (#363)

We allow array type declarations in both Java and C styles, e.g.
Java style: public static void main(String[] args)
C style:    public static void main(String args[])

However, Java style is preferred and we should only use this style
in our project.

Let's add ArrayTypeStyle rule to ensure that only the Java style
of array type declarations is used.

Author: eugenepeh

config/checkstyle/checkstyle.xml

@@ -155,6 +155,12 @@
     LENGTH and CODING CHECKS
     -->
 
+    <!-- Checks that array type declarations follow Java Style
+      Java style: public static void main(String[] args) // Allowed
+      C style:    public static void main(String args[]) // Not allowed
+    -->
+    <module name="ArrayTypeStyle"/>
+
     <module name="LineLength">
       <!-- Checks if a line is too long. -->
       <property name="max" value="120"/>