Add typings for is-callable (#27859)

* [@types/is-callable] Add type definition

* [@types/is-callable] Remove optional quantifier from parameter

types/is-callable/index.d.ts

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+// Type definitions for is-callable 1.1
+// Project: https://github.com/ljharb/is-callable
+// Definitions by: Daniel <https://github.com/nieltg>
+// Definitions: https://github.com/DefinitelyTyped/DefinitelyTyped
+
+declare function isCallable(val: any): val is (...args: any[]) => any;
+
+export = isCallable;

types/is-callable/is-callable-tests.ts

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+import isCallable = require("is-callable");
+
+isCallable(undefined);
+isCallable(null);
+isCallable(false);
+isCallable(true);
+isCallable([]);
+isCallable({});
+isCallable(/a/g);
+isCallable(new RegExp("a", "g"));
+isCallable(new Date());
+isCallable(42);
+isCallable(NaN);
+isCallable(Infinity);
+isCallable("foo");
+isCallable(Object("foo"));
+
+function func1() {}
+if (isCallable(func1)) {
+    func1();
+}
+
+const func2 = (x: number) => x * x;
+if (isCallable(func2)) {
+    func2(0);
+}

types/is-callable/tsconfig.json

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

types/is-callable/tslint.json

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