Add issue, feature proposals and PR template

[qarmin]

For now are empty, but sometimes providing e.g. OS version, bevy version, minimal project to reproduce is very helpful to reproduce bugs.

There should be also different templates for issues and feature proposals.

https://github.com/godotengine/godot/tree/master/.github/ISSUE_TEMPLATE

[lberrymage]

It would also be nice to have this to prevent needing PRs like #443 and #497, which only kept the changelog up-to-date for a short while. There have been a few changes since then and none of them have a changelog entry, probably because people just don't think to add one.

At least adding a pull request template with

• - I have added an entry to the changelog if necessary

would be beneficial on that front.

It'll also worth considering the "automatically add a label" feature of these templates for basic labels like "bug" and "enhancement".