chore(doc-gen): put typescript stuff into its own package

This means that we can now run just the d.ts file generation by running:

```bash
gulp docs/typings
```

In addition the type definition generation was messing with the other docs tasks
so separating it also fixes problems there.

docs/docs-package/mocks/testSrc.ts

@@ -0,0 +1,34 @@
+/**
+ * @module
+ * @description
+ * This is the module description
+ */
+
+export * from 'importedSrc';
+
+/**
+ * This is some random other comment
+ */
+
+/**
+ * This is MyClass
+ */
+export class MyClass {
+  message: String;
+
+  /**
+   * Create a new MyClass
+   * @param {String} name The name to say hello to
+   */
+  constructor(name) { this.message = 'hello ' + name; }
+
+  /**
+   * Return a greeting message
+   */
+  greet() { return this.message; }
+}
+
+/**
+ * An exported function
+ */
+export var myFn = (val: number) => return val * 2;