Add types and ExternalOperator, and modify parser accordingly

src/qir/ast/QIRLambda.java

src/qir/ast/operator/ExternalOperator.java

src/qir/parser/Parser.frame

src/qir/parser/Parser.java

src/qir/parser/Scanner.java

src/qir/parser/qir.atg

src/qir/parser/test.qir

src/qir/types/FunctionType.java

src/qir/types/QIRConstantType.java

src/qir/types/QIRType.java