Detect PRQL based on language ID rather than file extension

[jiripospisil]

Fixes #24.

[max-sixty]

Looks great to me! Thanks @jiripospisil !

When saving a .prql file, I can confirm it sets the lang to PRQL.

With the proposed code, until the lang is set — either explicitly or with a .prql extension — the output panel won't work — which is probably correct, otherwise it's going to try and parse lots of non-PRQL files.

[max-sixty]

@jiripospisil I'm going to hit the button if that's OK?