github CI: add PR

[vtorri]

Add PR support for CI, like:

on:
  push:
    branches: [ master ]
  pull_request:
    branches: [ master ]
    types: [opened, synchronize, reopened]

this allows to check if a PR compiles or not before pushing it. Imho, it's a very important feature to check is a PR does not break compilation or tests

do you want a PR ?

[vtorri]

no comment on this ?

[Ignacy Koper]

@vtorri they're using SourceHut Builds instead, see .builds folder. It's easy to connect it with GitHub to see the builds status.

[Michael Forney]

Sure, feel free to send a PR.

[Ryan Carsten Schmidt]

The PR for this is #98.