Add Cypress test suite

[wetneb]

Demonstrate how to test an extension while it's running in OpenRefine with Cypress.

[SoryRawyer]

These are two good starting points for putting together a Cypress test suite scaffold:

• the extension documentation page now has some guidance on this
• the Cypress suite in CommonsExtension and script for running Cypress in CI.

[SoryRawyer]

It doesn't look like I can assign myself, but I've been spending some time on this over the past week. I'm planning to open a PR that adds a basic test suite that runs OpenRefine with the extension installed. The example test I was going to include would navigate to the extension page and verify that the extension is listed on the installed extensions page.

[tfmorris]

It doesn't look like I can assign myself,

There weren't any teams assigned to the repo. I've added the committers team with write access, so hopefully you should be able to assign yourself now.