Feat greek letters and infinity

[MANDRW]

This pull request extends the math expression parser and visitor framework to support Greek letter tokens and infinity, as well as unary plus and minus operators. It also standardizes import paths for ANTLR-generated classes. These changes improve the parser's ability to handle mathematical notation more naturally and in a way that's closer to LaTeX.

Parser and Grammar Enhancements:

• Added GREEK and INFINITY tokens to MathLexer.g4, enabling recognition of Greek letter names (both lowercase and uppercase) and various representations of infinity.
• Updated MathParser.g4 to support unary plus/minus operators, Greek letter tokens, and infinity as valid expressions.

Visitor and Output Handling:

• Implemented visitUnarySign in AlgebraVisitor to handle unary plus and minus expressions.
• Implemented visitInfinity in BasicVisitor to output \infty for infinity tokens.
• Integrated a new GreekVisitor in MainVisitor and routed Greek and infinity expressions to the appropriate visitors.