update: Severity to usize

Signed-off-by: Jia Chao <jiachao2130@126.com>

src/lib.rs

@@ -87,6 +87,18 @@ impl FromStr for Severity {
     }
 }
 
+impl From<&Severity> for usize {
+    fn from(item: &Severity) -> usize {
+        match *item {
+            Severity::None => 0,
+            Severity::Low => 1,
+            Severity::Medium => 2,
+            Severity::High => 3,
+            Severity::Critical => 4,
+        }
+    }
+}
+
 // 定义 ParseSeverityError 枚举类型来表示解析错误
 #[derive(Debug, Clone)]
 pub enum ParseSeverityError {