Add issue and PR templates

[Jondolf]

What problem does this solve or what need does it fill?

This repository doesn't have any issue or PR templates. For consistency and to uphold a certain bar of quality for issues, basic templates could be very useful.

What solution would you like?

Add templates. This issue's structure is an example of what an issue template could be (inspired by one of Bevy's issue templates).

What alternative(s) have you considered?

Keep it as is, without any default structure or guidance.