npm/qsharp - Make languageFeatures an optional parameter again

[minestarks]

Revert breaking change for QCOM. I don't recall the specific reason we had to make this required - I think it was an accident that snuck in when we made the languageFeatures a required method in OTHER method signatures in this file (#2078)

[billti]

The quoted PR where this broke seems like it was a deliberate change... but I don't see where this revert would break anything. If you've tested and looks good, then that's good with me.

[minestarks]

Just going over the details for my own sanity: There were a few other arguments also called languageFeatures, that were changed to be required arguments, and those were deliberate, but this one must have got lumped in by accident. @idavis and I had discussed and decided it was ok to make languageFeatures optional on the getAst and getHir methods (discussed in this comment) This was not breaking since QCOM doesn't use those methods. What I missed in the final revision is that there was another languageFeatures property on ProgramConfig that was made required, and that was the breaking change. 😭

It ended up being nbd (QCOM has long been fixed) - but I wanted to get this PR out for hygiene. It's just a better API to leave that property optional.