Add polyfill to fix context-menu extension in Safari

[emilykl]

About

The cytoscape-context-menu extension causes a breaking "Illegal Constructor" error in Safari (see issue here).

This can be fixed by adding an existing polyfill to the project.

Description of changes

This PR adds the necessary polyfill to fix the "Illegal Constructor" error in Safari.

Pre-Merge checklist

• The project was correctly built with npm run build:all.
• If there was any conflict, it was solved correctly.
• All changes were documented in CHANGELOG.md.
• All tests on CircleCI have passed.
• All Percy visual changes have been approved.
• Two people have 💃'd the pull request. You can be one of these people if you are a Dash Cytoscape core contributor.

Steps to test

1. Check out branch
2. Run npm install && npm run build:all
3. Run the context menu demo Dash app: python demos/usage-context-menu.py and navigate to http://127.0.0.1:8050 in Safari
4. If the app loads properly without an "Illegal Constructor" Javascript error, all is good.

[alexcjohnson]

💃

[cleaaum]

Works on my machine on safari!