Conform Identifier to Sendable

This isn't explicitly needed for this problem but it seems as though
the default for all types that can conform to Sendable should

This also sets up this new type for Swift 6.0 (and the current Swift 5.10)
to pass this type around safely when needed

Sources/SwiftSyntax/Identifier.swift

@@ -13,7 +13,7 @@
 import Foundation
 
 /// An abstraction for sanitized values on a token.
-public struct Identifier: Equatable {
+public struct Identifier: Equatable, Sendable {
   /// The sanitized `text` of a token.
   public let name: String