chore(ci): Separate workflows by workflow trigger, rename config files

[bsmth]

The "Label PRs by size" workflow relies on the pull_request event type, but we're using pull_request_target, so the job is successful, but does nothing:

Event is not a pull request, doing nothing

Instead of interleaving the event types:

on:
  - pull_request
  - pull_request_target

jobs:
  label:
    steps:
      - if: github.event_name == 'pull_request_target'
# etc.

I'm putting these two jobs in their own files for simplicity.