create PR template to contain a reviewer checklist

[PhilBastian]

I've included a few checklist items that I think are good for reviewers to check for. Feel free to suggest others, but they shouldn't be too onerous or be wishful thinking for things which aren't true already (e.g. I intentionally didn't include a checkbox for "any added or changed code is covered by tests")

[cquirosj]

I think this is a solid first version. In a future iteration, it could be helpful to add an item to the checklist pointing to guidance on when to use “composables” vs plain utility functions; for example, explaining that composables typically follow the use naming convention and leverage Vue’s composition-API (reactivity, lifecycle hooks, etc.), whereas a regular utility should remain a simple, stateless function intended purely to avoid duplication.

[johnsimons]

lgtm