Implement a way to exclude PRs/issues in autolabeling

[JohnTitor]

So, the wg-prioritization usually doesn't prioritize PRs. But autolabeling adds the labels regardless of issues/PRs.
It'd be great if we could exclude PRs (for this purpose, excluding issues is unnecessary but it should be done in the same PR easily) in the config, I think.

[apiraino]

adding an example for context: rust-lang/rust#78151 has been wrongly pulled into this Zulip thread