Move all symbolName logic into one method

[IanDelMar]

• Merged symbolName logic from enterNode() and getNodeName() into getSymbolName() for improved readability and maintainability.
• Added assertions to detect nodes without names, ensuring issues are caught early.
• Renamed $fullSymbolName to $symbolName for consistency, as we only handle "full" symbol names.
• Changed the fullSymbolName attribute to WPStubs_symbolName to minimize the risk of conflicts with existing attributes or potential overrides when extending this visitor, even though such issues are unlikely.

[szepeviktor]

Too many bullet points!! 🙃