Add known TypeScript ESLint extensions

Author: Christopher Quadflieg

src/extends/index.d.ts

@@ -1,10 +1,11 @@
 import type { LiteralUnion } from '../utility-types';
 import type { EslintExtensions } from './eslint';
+import type { TypescriptEslintExtensions } from './typescript-eslint';
 
 /**
  * All known extensions.
  */
-export type KnownExtensions = LiteralUnion<EslintExtensions>;
+export type KnownExtensions = LiteralUnion<EslintExtensions | TypescriptEslintExtensions>;
 
 /**
  * Extending Configuration Files.

src/extends/typescript-eslint.d.ts

@@ -0,0 +1,8 @@
+/**
+ * Typescript eslint extensions.
+ *
+ * @see [Typescript eslint extensions](https://github.com/typescript-eslint/typescript-eslint/blob/master/docs/getting-started/linting/README.md#configuration)
+ */
+export type TypescriptEslintExtensions =
+  | 'plugin:@typescript-eslint/recommended'
+  | 'plugin:@typescript-eslint/recommended-requiring-type-checking';