ci: add a ecosystem-ci-trigger workflow

[haoqunjiang]

Inspired by https://github.com/vitejs/vite/blob/main/.github/workflows/ecosystem-ci-trigger.yml with the following modifications:

• Allow all members of the vuejs organization to trigger the CI, instead of hard-coding the allowedUsers list
• Use a fine-grained personal access token instead of GitHub App, because we already have the vue-bot account, no need to create a new App.
  • The personal access token needs these permissions:
    • Actions: Read and Write
    • Pull requests: Read and Write
  • The token needs to be added to this repo & the ecosystem-ci repo's action secrets, in the name of ECOSYSTEM_CI_ACCESS_TOKEN
    • https://github.com/vuejs/core/settings/secrets/actions
    • https://github.com/vuejs/ecosystem-ci/settings/secrets/actions

Usage:
Add comment on any PR in the following format:

• /ecosystem-ci run to trigger a full run
• /ecosystem-ci run vitepress to trigger the CI for the specific suite (vitepress)