[FR] Github issue / PR templates and workflows

[or-shachar]

Copying from https://github.com/bazelbuild/rules_python/pull/334 by @alexeagle

See building-a-strong-community/using-templates-to-encourage-useful-issues-and-pull-requests

This will ask users filing an issue to indicate if it's a bug or a feature request and prompt them to include answers to common questions, reducing how many round-trips they need from maintainers. Same for pull requests, especially to think about whether this is a breaking change.