feat(nixf-tidy): flag `--severity`

[Malix-Labs]

Is your feature request related to a problem? Please describe.

It might be desirable to only output high severity reports

nixd/libnixf/include/nixf/Basic/Diagnostic.h

Lines 145 to 168 in 0e07c08

	/// The super class for all diagnostics.
	/// concret diagnostic types are defined in Diagnostic*.inc
	class Diagnostic : public PartialDiagnostic {
	public:
	/// Each diagnostic contains a severity field,
	/// should be "Fatal", "Error" or "Warning"
	/// this will affect the eval process.
	///
	/// "Warning" -- just a warning.
	enum Severity {
	/// shouldn't eval the code, e.g. parsing error.
	DS_Fatal,
	/// trigger an error in nix, but we can recover & eval the code.
	DS_Error,
	/// A warning.
	DS_Warning,
	/// An information.
	DS_Info,
	/// A hint. Hints are usually not rendered directly in some editor GUI
	/// So this is suitable for liveness analysis results.
	/// For example, "unused xxx"
	DS_Hint,
	};

Currently, nixf-tidy does not have a way to specify the minimum severity to output

Describe the solution you'd like

A flag --severity <SEVERITY> in nixf-tidy to enable it to output reports above a provided security threshold

The <SEVERITY> argument should be a valid enum Severity value :

nixd/libnixf/include/nixf/Basic/Diagnostic.h

Lines 145 to 168 in 0e07c08

	/// The super class for all diagnostics.
	/// concret diagnostic types are defined in Diagnostic*.inc
	class Diagnostic : public PartialDiagnostic {
	public:
	/// Each diagnostic contains a severity field,
	/// should be "Fatal", "Error" or "Warning"
	/// this will affect the eval process.
	///
	/// "Warning" -- just a warning.
	enum Severity {
	/// shouldn't eval the code, e.g. parsing error.
	DS_Fatal,
	/// trigger an error in nix, but we can recover & eval the code.
	DS_Error,
	/// A warning.
	DS_Warning,
	/// An information.
	DS_Info,
	/// A hint. Hints are usually not rendered directly in some editor GUI
	/// So this is suitable for liveness analysis results.
	/// For example, "unused xxx"
	DS_Hint,
	};

To keep backward compatibility and sane defaults, the default <SEVERITY> should be the minimum, which would output every reports (currently, DS_Hint)

Describe alternatives you've considered

No response

Additional context

• feat(nixf-tidy): --ci flag #840