simplify code contributions via fully automating the dev setup.

[nisarhassan12]

Hi.

I work for Gitpod. This PR adds gitpod config to the repo to make contributions easier for newcomers i.e with a single click it will launch a workspace and automatically:

• clone the fullcalendar repo.
• install the dependencies.
• run yarn build.
• run yarn watch.

This can be helpful for beginners i.e they can start coding instantly without setting anything up.

You can give it a try on my fork of the repo via the following link:

https://github.com/nisarhassan12/fullcalendar

This is how it looks:

[acerix]

Please submit a ticket under Issues explaining this feature, then link this PR to it.