Added KnownKeys mapped type

Author: Ben Grynhaus

libs/core/src/lib/declarations/known-keys.d.ts

@@ -0,0 +1,22 @@
+/**
+ * Get the known keys (i.e. no index signature) of T.
+ *
+ * @example
+```typescript
+interface Options {
+  key: string;
+  title: string;
+  [dataProperty: string]: string | number;
+}
+
+type KnownKeysOfOptions = KnownKeys<Options>; // 'key' | 'title';
+// (vs. type KeysOfOptions = keyof Options // string | number;)
+```
+
+ * Taken from https://stackoverflow.com/questions/51465182/typescript-remove-index-signature-using-mapped-types
+ */
+export type KnownKeys<T> = { [K in keyof T]: string extends K ? never : number extends K ? never : K } extends {
+  [_ in keyof T]: infer U
+}
+  ? U
+  : never;

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

@@ -1,5 +1,6 @@
 // Copyright (c) Microsoft Corporation. All rights reserved.
 // Licensed under the MIT License.
 
+export * from './known-keys';
 export * from './omit';
 export * from './StringMap';