pull_request_target & permissions

[jmnote]

This PR is relatied to the issue #33

I found a best practice that resolves the issue.
https://github.com/actions/labeler/blob/a11f1c41635ff1796dffb74e7fa24cbcb92f66cf/README.md#create-workflow

The old workflow file no longer works.
To make it work fine, we need to change the event and add some permissions.

name: size-label
## on: pull_request
on: pull_request_target
jobs:
  size-label:
    permissions: ## added
      contents: read
      pull-requests: write
    runs-on: ubuntu-latest
    steps:
      - name: size-label
        uses: "pascalgn/[email protected]"
        env:
          GITHUB_TOKEN: "${{ secrets.GITHUB_TOKEN }}"

[pascalgn]

Thanks for the PR, it's merged now! 👍