First-party Typescript support

[maxkfranz]

Description of new feature

What should the new feature do? For visual features, include an image/mockup of the expected output.

Support first-party typings in the main Cytoscape.js repo.

Motivation for new feature

Describe your use case for this new feature.

• Improved support for Typescript projects.
• Make it easier for third-party contributors to improve the typings. Just make a pull request in this repo.

See also:

• TypeScript Definition for Cytoscape.js - It is being worked on now! #1012

For reviewers

Reviewers should ensure that the following tasks are carried out for incorporated issues:

• Ensure that the reporter has adequately described their idea. If not, elicit more information about the use case. You should iteratively build a spec together.
• Ensure that the issue is a good fit for the core library. Some things are best done in extensions (e.g. UI-related features that aren't style-related). Some things are best done by app authors themselves -- instead of in Cytoscape libraries.
• The issue has been associated with a corresponding milestone.
• The commits have been incorporated into the unstable branch via pull request. The corresponding pull request is cross-referenced.

[maxkfranz]

PR up in #3308