OpenQASM 'syntax-only' mode

Now that we have an excellent OpenQASM parser, we should make our QDK extension the best it can be for folks that just want to write OpenQASM code even if not using the rest of our stack (i.e., don't give errors or other behaviors related to things beyond parsing).

[Prototype](https://github.com/microsoft/qsharp/tree/iadavis/qasm-spec-mode) works and disables all LS integrations. Setting name needs to be discussed.