Update issue templates (#23296)

Summary:
Go back to simpler issue template titles, with emoji. The goal is to make it easier, at a glance, to pick the right template.

We also add a new "regression" template that will automatically label the issue as a regression.

Other changes:
- Added Christoph as a CODEOWNER for package.json and markdown file changes. Usually, any PR that touches package.json has to be imported manually by a Facebook employee.

[General] [Changed] - Updated GitHub issue templates.
Pull Request resolved: https://github.com/facebook/react-native/pull/23296

Differential Revision: D13974378

Pulled By: hramos

fbshipit-source-id: d8962a84397c16e7556ac167a3505f7c7d725915