Add automatic dependency updating workflow to github actions

[Cawllec]

Adds a workflow triggered by remote branches that will:

• Run an appropriate update script
• Push and create a PR into next
• Use an appropriate commit message to trigger the full CI run

This will require some upstream changes before it starts triggering, but has been extensively tested in the forked repo.

Note - This is an internal copy of #2168 as I wanted CI to run against it

[github-actions]

@bugsnag/browser bundle size diff

	Minified	Minfied + Gzipped
Before	44.21 kB	13.51 kB
After	44.21 kB	13.51 kB
±	No change	No change

code coverage diff

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Generated by 🚫 dangerJS against 58fbc1e

[gingerbenw]

did we use an environment variable for the reviewer in another PR?

[Cawllec]

Yes indeed