Export TextLayerRenderTask type

[jerry1100]

Attach (recommended) or Link to PDF file here: not relevant

Configuration:

• Web browser and its version: not relevant
• Operating system and its version: not relevant
• PDF.js version: 2.14.305
• Is a browser extension: no

Steps to reproduce the problem:

1. In a webpack project, try to import TextLayerRenderTask from pdfjs-dist

What is the expected behavior? (add screenshot)
TextLayerRenderTask be exported

What went wrong? (add screenshot)

Link to a viewer (if hosted on a site other than mozilla.github.io/pdf.js or as Firefox/Chrome extension): n/a

We need TextLayerRenderTask to be exported for our TypeScript project. Currently, we're just polyfilling the type but it'd be great if this could be officially exported.

[jerry1100]

I tried to submit a PR, but couldn't figure out how to actually export the type. Here's what I tried:

1. Export TextLayerRenderTask from text_layer.js

2. Run gulp types to build the types

However, pdf.d.ts still doesn't have TextLayerRenderTask.

I'm happy to submit a PR if someone could help me figure out how to export the type.