Explicit function declaration nodes

Date: 2026-05-20 Area: parser

Function declarations use explicit statement nodes. Replaced the temporary IsFunctionDeclaration flag on TGocciaVariableDeclaration with TGocciaFunctionDeclaration and TGocciaExportFunctionDeclaration, while keeping function expressions on TGocciaFunctionExpression. This keeps declaration paths readable and maintainable, and gives parser, interpreter, bytecode, export, hoisting, and redeclaration logic exact AST semantics instead of inferring declaration intent from variable-declaration shape.