feat(is-gif): new definition (#47153)

Small util used by imagemin:
- definition file
- tests

https://github.com/sindresorhus/is-gif

Thanks!

types/is-gif/index.d.ts

@@ -0,0 +1,12 @@
+// Type definitions for is-gif 3.0
+// Project: https://github.com/sindresorhus/is-gif#readme
+// Definitions by: Piotr Błażejewicz <https://github.com/peterblazejewicz>
+// Definitions: https://github.com/DefinitelyTyped/DefinitelyTyped
+/// <reference types="node" />
+
+/**
+ * Returns a boolean of whether input is a GIF image.
+ */
+declare function isGif(input: Buffer | Uint8Array): boolean;
+
+export = isGif;

types/is-gif/is-gif-tests.ts

@@ -0,0 +1,4 @@
+import isGif = require('is-gif');
+
+isGif(Buffer.from('x')); // $ExpectType boolean
+isGif(new Uint8Array(Buffer.from('x'))); // $ExpectType boolean

types/is-gif/tsconfig.json

@@ -0,0 +1,23 @@
+{
+    "compilerOptions": {
+        "module": "commonjs",
+        "lib": [
+            "es6"
+        ],
+        "noImplicitAny": true,
+        "noImplicitThis": true,
+        "strictFunctionTypes": true,
+        "strictNullChecks": true,
+        "baseUrl": "../",
+        "typeRoots": [
+            "../"
+        ],
+        "types": [],
+        "noEmit": true,
+        "forceConsistentCasingInFileNames": true
+    },
+    "files": [
+        "index.d.ts",
+        "is-gif-tests.ts"
+    ]
+}

types/is-gif/tslint.json

@@ -0,0 +1 @@
+{ "extends": "dtslint/dt.json" }