ci: add Gravity to CI

[oscard0m]

Hi! I'm @oscard0m, and I work at Mainmatter.

We really appreciate the great work you're doing at mswjs/mswjs.io! We thought Gravity could be useful to the project.

What is Gravity?

Gravity is a free tool that helps maintainers stay on top of asset size increases as part of their CI pipeline. Gravity catches potentially unintended or disproportionate growth of assets/bundle sizes that result out of code changes in a PR before they hit production, e.g. added dependencies that grow the bundle size disproportionally. Without Gravity, it's hard for maintainers to even be aware of these changes, and bundle/assets sizes often just keep growing unnoticed.

Here's a quick video showing Gravity in action:

https://youtu.be/2vD_geF_Ask

You can read a more detailed explanation in our
blog post.

We also prepared an example PR with Gravity set up on a fork of mswjs/mswjs.io: mainmatter#1 and a couple of PR examples once this is set up: mainmatter#2 and mainmatter#3 where Gravity catches a bundle size increase:

Last steps

This PR introduces Gravity to the project, but it is expected the maintainers
take some extra actions to connect all the dots:

1. Install Gravity GitHub App to your organization.
2. Once installed, create a new Gravity project in https://gravity.ci.
3. Set up the Gravity token for your repo and add the Gravity job to your GitHub Actions workflow.

[vercel]

@oscard0m is attempting to deploy a commit to the MSW Team on Vercel.

A member of the Team first needs to authorize it.