Added extern function declarations

CMakeLists.txt

examples/funcDecl.dusk

grammar.txt

include/dusk/AST/ASTVisitor.h

include/dusk/AST/DiagnosticsParse.h

include/dusk/AST/Stmt.h

include/dusk/Basic/TokenDefinition.h

include/dusk/Frontend/Compiler.h

include/dusk/IRGen/IRGenerator.h

include/dusk/Parse/Parser.h

include/dusk/Parse/Token.h

lib/AST/ASTPrinter.cpp

lib/AST/ASTWalker.cpp

lib/AST/Stmt.cpp

lib/Frontend/Compiler.cpp

lib/IRGen/GenStmt.cpp

lib/IRGen/IRGenerator.cpp

lib/Parser/Lexer.cpp

lib/Parser/ParseDecl.cpp

lib/Parser/ParseStmt.cpp