Add PR templates
Created by: felixfbecker
#3166 (closed) happened because both Loic and I forgot to test on Firefox. In addition, for every browser extension PR I manually type a checklist to enumerate testing for all code hosts. In the browser extension repo we used to have a PR template to remind of this and save typing.
This adds back such a template. Upon opening a PR, we will be able to choose which PR template to chose. I added
- one for the browser extension
- one for the current template with the changelog reminder and test plan
- one for everything else that does not need changelog/testing (docs, tooling, ...)