Can we call nix-instantiate without --parse?

[itsfarseen]

In linter.ts#L95, we call nix-instantiate with the --parse argument.
If we omit it, we will also get diagnosis for things like pkgs.foozz doesn't exist.
Currently it only analyzes syntatical errors like misplaced semicolons/parens etc and missing top-level variables.

Other than the fact that omiting --parse is slow, is there any other reason we can't use that?
nix-instantiate takes 1.4s to execute on my machine. Whereas, nix-instantiate --parse only takes 0.1s.

If there are no other issues, I think I'll be able to do a PR that runs nix-instantiate --parse first, and then fires off nix-instantiate in the background and update the output once it completes.

[jnoortheen]

I will be glad to accept the PR if it is opt-in while explaining the tradeoffs in the settings.