chore: add Readonly<T> type helper

Author: JairusSW

src/ast.ts

@@ -177,6 +177,13 @@ export abstract class Node {
     return new NamedTypeNode(Node.createSimpleTypeName("", range), null, false, range);
   }
 
+  static createReadonlyType(
+    innerType: TypeNode,
+    range: Range
+  ): NamedTypeNode {
+    return new NamedTypeNode(Node.createSimpleTypeName("Readonly", range), [ innerType ], false, range);
+  }
+
   static createTypeParameter(
     name: IdentifierExpression,
     extendsType: NamedTypeNode | null,

src/parser.ts

@@ -519,12 +519,7 @@ export class Parser extends DiagnosticEmitter {
     if (token == Token.Readonly) {
       let innerType = this.parseType(tn, acceptParenthesized, suppressErrors);
       if (!innerType) return null;
-      type = Node.createNamedType(
-        Node.createSimpleTypeName("Readonly", tn.range(startPos, tn.pos)),
-        [ innerType ],
-        false,
-        tn.range(startPos, tn.pos)
-      );
+      type = Node.createReadonlyType(innerType, tn.range(startPos, tn.pos));
 
     // '(' ...
     } else if (token == Token.OpenParen) {