Added return type support for function in parser and lexer

examples/funcDecl.dusk

examples/globExpr.dusk

grammar.txt

include/dusk/AST/ASTVisitor.h

include/dusk/AST/CMakeLists.txt

include/dusk/AST/Decl.h

include/dusk/AST/DiagnosticsParse.h

include/dusk/AST/Stmt.h

include/dusk/AST/Type.h

include/dusk/Basic/TokenDefinition.h

include/dusk/IRGen/Context.h

include/dusk/Parse/Parser.h

include/dusk/Parse/Token.h

lib/AST/ASTPrinter.cpp

lib/AST/ASTWalker.cpp

lib/AST/CMakeLists.txt

lib/AST/Decl.cpp

lib/AST/Type.cpp

lib/Frontend/Compiler.cpp

lib/IRGen/Context.cpp