Fix throwing ArrayIndexOutOfBoundsException bug

Requirements to run

• JDK 11

Requirements to build

• JDK 21

Changed

• Update build system
• Modify the exception detail message to include whether the operator is unary or binary when an unknown operator is detected

Fixed

• Fix an issue where IntExpr.eval(String) would throw ArrayIndexOutOfBoundsException for expressions like:
  • Binary operators missing right-hand operand (e.g., "1+")
  • Unary operators missing operand (e.g., "-")
• Fix Javadoc and Checkstyle warnings

Initial release

Requirements to run

• JDK 11

Requirements to build

• JDK 17

New

• Add APIs to evaluate integer expressions and to visualize the syntax trees representing the expressions in Reverse Polish notation.