checkstyle.xml: add 3 modules

Author: stephengold

config/checkstyle/checkstyle.xml

@@ -95,7 +95,11 @@
         <module name="FinalClass"/>
         <module name="HideUtilityClassConstructor"/>
         <module name="InterfaceIsType"/>
+        <module name="MutableException"/>
         <module name="OneTopLevelClass"/>
+        <module name="ThrowsCount">
+            <property name="max" value="1"/>
+        </module>
 
         <!-- Checks for common coding problems:       -->
         <!-- See https://checkstyle.org/checks/coding -->
@@ -105,6 +109,7 @@
         <module name="CovariantEquals"/>
         <module name="DefaultComesLast"/>
         <module name="EmptyStatement"/>
+        <module name="EqualsAvoidNull"/>
         <module name="EqualsHashCode"/>
         <module name="FallThrough"/>
         <module name="HiddenField">