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

src/display/canvas.js

@@ -430,6 +430,9 @@ var CanvasExtraState = (function CanvasExtraStateClosure() {
   return CanvasExtraState;
 })();
 
+/**
+ * @type {any}
+ */
 var CanvasGraphics = (function CanvasGraphicsClosure() {
   // Defines the time the executeOperatorList is going to be executing
   // before it stops and shedules a continue of execution.