Add typescript definitions

This PR adds typescript definitions from the JSDoc already present.
It adds a new gulp-target 'types' that calls 'tsc', the typescript
compiler, to create the definitions.

To use the definitions, users can simply do the following:

```
import {getDocument, GlobalWorkerOptions} from "pdfjs-dist";
import pdfjsWorker from "pdfjs-dist/build/pdf.worker.entry";
GlobalWorkerOptions.workerSrc = pdfjsWorker;

const pdf = await getDocument("file:///some.pdf").promise;
```

Co-authored-by: @oBusk
Co-authored-by: @tamuratak

test/types/main.ts

@@ -0,0 +1,20 @@
+import { getDocument } from "pdfjs-dist";
+
+class MainTest {
+  task: ReturnType<typeof getDocument> | undefined;
+
+  constructor(public file: string) {
+  }
+
+  loadPdf() {
+    this.task = getDocument("file://" + this.file);
+    return this.task.promise;
+  }
+}
+
+// This is actually never called, as the test only consists in compiling the file.
+// The compilation will crawl through all files and make sure that the types are consistent.
+const mt = new MainTest("../pdfs/basicapi.pdf");
+mt.loadPdf().then(() => {
+  console.log("loaded");
+});

test/types/tsconfig.json

@@ -0,0 +1,26 @@
+{
+  "compilerOptions": {
+    "outDir": "../../build/tmp",
+    "sourceMap": true,
+    "declaration": false,
+    "moduleResolution": "node",
+    "emitDecoratorMetadata": true,
+    "experimentalDecorators": true,
+    "target": "es5",
+    "module": "es2015",
+    "baseUrl": "./",
+    "strict": true,
+    "types": [],
+    "lib": [
+      "es2017",
+      "dom"
+    ],
+    "paths": {
+      "pdfjs-dist": ["../../build/typestest"],
+      "pdfjs-dist/*": ["../../build/typestest/build/*"]
+    }
+  },
+  "files": [
+    "main.ts"
+  ],
+}