Create definition for just-throttle (#38707)

* Create definition for just-throttle

* Use more specific URL for the project

* Improve comments in tests slightly

types/just-throttle/index.d.ts

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+// Type definitions for just-throttle 1.1
+// Project: https://github.com/angus-c/just/tree/master/packages/function-throttle
+// Definitions by: Dominik Rowicki <https://github.com/papermana>
+// Definitions: https://github.com/DefinitelyTyped/DefinitelyTyped
+// TypeScript Version: 3.0
+
+declare function throttle(fn: (...args: unknown[]) => unknown, interval: number, callFirst?: false): () => void;
+declare function throttle<T>(fn: T, interval: number, callFirst?: true): T;
+
+export = throttle;

types/just-throttle/just-throttle-tests.ts

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+import throttle = require('just-throttle');
+
+// returns a function that sets an interval but doesn't do anything else by
+// default
+throttle(() => 'foo', 200); // $ExpectType () => void
+throttle(() => 'foo', 200, false); // $ExpectType () => void
+
+// if third argument is true, the returned function will immediately apply the
+// callback when called
+throttle(() => 'foo', 200, true); // $ExpectType () => string
+throttle((foo: string) => 42, 200, true); // $ExpectType (foo: string) => number

types/just-throttle/tsconfig.json

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+{
+    "compilerOptions": {
+        "module": "commonjs",
+        "lib": [
+            "es6"
+        ],
+        "noImplicitAny": true,
+        "noImplicitThis": true,
+        "strictFunctionTypes": true,
+        "strictNullChecks": true,
+        "baseUrl": "../",
+        "typeRoots": [
+            "../"
+        ],
+        "types": [],
+        "noEmit": true,
+        "forceConsistentCasingInFileNames": true
+    },
+    "files": [
+        "index.d.ts",
+        "just-throttle-tests.ts"
+    ]
+}

types/just-throttle/tslint.json

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+{ "extends": "dtslint/dt.json" }