Add a reusable addtomainproject.yml workflow

[swashbuck]

@taylortom Are you planning to add a reusable addtomainproject.yml to this repo (alongside release.yml)?

Right now, each adaptlearning repo has its own copy of the workflow, with the project board URL hardcoded. I built a parameterized workflow_call version for cgkineo that takes a project-url input (defaults per-org, overridable per-repo) since we run several boards. Happy to PR it into your repo if useful as it would let any org/board reuse it.

Example that would default to https://github.com/orgs/adaptlearning/projects/2 for the project board:

name: Add to Project Workflow
on:
  workflow_call:
    inputs:
      project-url:
        description: 'GitHub project URL'
        required: false
        type: string
        default: 'https://github.com/orgs/adaptlearning/projects/2'
jobs:
  add-to-project:
    name: Add to main project
    runs-on: ubuntu-latest
    steps:
      - uses: actions/add-to-project@v0.1.0
        with:
          project-url: ${{ inputs.project-url }}
          github-token: ${{ secrets.ADDTOPROJECT_TOKEN }}