move common identifier validation logic out of Prerelease.swift

It’s used for more than validating prerelease.

Author: WowbaggersLiquidLunch

Sources/SymbolKit/SymbolGraph/SemanticVersion/Identifier Validation.swift

@@ -0,0 +1,21 @@
+/*
+ This source file is part of the Swift.org open source project
+ 
+ Copyright (c) 2022 Apple Inc. and the Swift project authors
+ Licensed under Apache License v2.0 with Runtime Library Exception
+ 
+ See https://swift.org/LICENSE.txt for license information
+ See https://swift.org/CONTRIBUTORS.txt for Swift project authors
+ */
+
+extension Character {
+    /// A Boolean value indicating whether this character is allowed in a semantic version's identifier.
+    internal var isAllowedInSemanticVersionIdentifier: Bool {
+        isASCII && (isLetter || isNumber || self == "-")
+    }
+    
+    /// A Boolean value indicating whether this character is allowed in a semantic version's numeric identifier.
+    internal var isAllowedInSemanticVersionNumericIdentifier: Bool {
+        isASCII && isNumber
+    }
+}

Sources/SymbolKit/SymbolGraph/SemanticVersion/Prerelease.swift

@@ -142,17 +142,3 @@ extension SymbolGraph.SemanticVersion.Prerelease.Identifier: CustomStringConvert
         }
     }
 }
-
-// MARK: - Validating Identifiers
-
-extension Character {
-    /// A Boolean value indicating whether this character is allowed in a semantic version's identifier.
-    internal var isAllowedInSemanticVersionIdentifier: Bool {
-        isASCII && (isLetter || isNumber || self == "-")
-    }
-    
-    /// A Boolean value indicating whether this character is allowed in a semantic version's numeric identifier.
-    internal var isAllowedInSemanticVersionNumericIdentifier: Bool {
-        isASCII && isNumber
-    }
-}