Add IndexSignature mapped type

Author: Ben Grynhaus

libs/core/src/lib/declarations/index-signature.d.ts

@@ -0,0 +1,15 @@
+/**
+ * Gets the index signature of a type.
+ *
+ * @example
+ ```typescript
+interface Options {
+  key: string;
+  title: string;
+  [dataProperty: string]: string | number;
+}
+
+type IndexOfOptions = IndexSignature<Options>; // { [x: string]: string | number; [x: number]: string | number; }
+```
+ */
+export type IndexSignature<T> = Pick<T, keyof T>;

libs/core/src/lib/declarations/public-api.d.ts

@@ -1,6 +1,7 @@
 // Copyright (c) Microsoft Corporation. All rights reserved.
 // Licensed under the MIT License.
 
+export * from './index-signature';
 export * from './known-keys';
 export * from './omit';
 export * from './StringMap';