Allow custom derivate registration for _modify accessors

Author: asl

Sources/SwiftParser/Attributes.swift

@@ -650,7 +650,7 @@ extension Parser {
     let unexpectedBeforeAccessor: RawUnexpectedNodesSyntax?
     let accessor: RawTokenSyntax?
     if period != nil {
-      (unexpectedBeforeAccessor, accessor) = self.expect(.keyword(.get), .keyword(.set), default: .keyword(.get))
+      (unexpectedBeforeAccessor, accessor) = self.expect(.keyword(.get), .keyword(.set), .keyword(._modify), default: .keyword(.get))
     } else {
       (unexpectedBeforeAccessor, accessor) = (nil, nil)
     }