Set up sponsorkit for sponsors image generation

[yanthomasdev]

This pull request adds sponsorkit to help generate sponsor images we can link in our READMEs. I've configured it in the sponsorkit.config.ts file based on our OpenCollective tiers.

For the action to work, we'll need to add two tokens I've specified in the scheduler.yml workflow:

env:
  # Requires no scopes or 2FA actions enabled.
  SPONSORKIT_OPENCOLLECTIVE_KEY: ${{ secrets.SPONSORKIT_OPENCOLLECTIVE_KEY }}
  # Requires the `read:user` and `read:org` scopes.
  SPONSORKIT_GITHUB_TOKEN: ${{ secrets.GITHUB_TOKEN }}

I've added sponsorkit here since it kinda made the most sense, but its OK to move this to a new or another repository if it feels weird to have package.json and TypeScript on a .github repo.