[process] Create a custom PR template

[pq]

The PR template we're inherting from flutter is mostly being disregarded. Let's consider crafting one that make more sense for this repo.

As an inspiration, we could look to Dart, which is far simpler but I think sends a good message:

https://github.com/dart-lang/sdk/blob/main/.github/PULL_REQUEST_TEMPLATE.md