Add bundlewatch to track bundle size changes in PRs

[davwheat]

Changes proposed in this pull request:
Add PR status to show the change in bundle size as a result of it. This CI workflow must run on master before it begins working.

It's also possible to define a max size for each of the JS files which will cause a failing status if the bundle is over that size, but I decided not to implement this.

For more configuration options, see: https://bundlewatch.io/#/reference/configuration

Example PR: https://github.com/davwheat/flarum-core/pull/8

Setup steps:

1. Head here to get auth ID: https://service.bundlewatch.io/setup-github
2. Create repo secret called BUNDLEWATCH_GITHUB_TOKEN with the token inside

Screenshot

[dsevillamartin]

What if we merged this action with the JS build? That way we could have the action be called JavaScript / Bundlewatch (unsure if you can have different actions with the same main name 🤔)

[davwheat]

@datitisev The main reason I opted not to is that we only run that workflow on pushes to master, and not for PRs and more. We don't want the bot committing dist JS onto PRs as that could create all sorts of merge conflicts.

[davwheat]

@datitisev Any objections before I merge?