Feature Request: Detect no_std per-crate and ignore check.allTargets

[DianaNites]

If possible, the option for RA to detect that a crate* is no_std and disable check.allTargets, to seamlessly prevent issues like #2142 once and for all, where you can either have RA checking tests where it makes sense, or you can stop bogus errors in no_std crates, but not both.

*specifically, in my case, I have a VSCode workspace with various cargo crates/workspaces, some no_std, some not, with per-folder .cargo/config.toml setting targets.

Alternatively, another option would be do detect no_std and use a new option, checkOnSave.extraNoStdArgs, for something like as a (manual) solution. Or both?

{
    "rust-analyzer.checkOnSave.allTargets": false,
	"rust-analyzer.checkOnSave.extraArgs": [
        "--all-targets"
    ]
    "rust-analyzer.checkOnSave.extraNoStdArgs": [
        "--bins"
    ]
}

[Veykril]

Doing this for flycheck (that is cargo check) is kind of tricky right now, as we built the command before we even analyze crates. We'd have to feed back this info after having analyzed all crates which we only do lazily ... So we probably want to change flycheck do built the command on demand.