add action that opens the PRs for us

[aduh95]

There are cases (e.g. when working on a release) where the current process is kinda slow, and we could speed it up by having the automation create the PR for us.

Having a workflow that can be triggered from the CLI would fix that.

# Create a PR for a post:
gh workflow run create-pr.yml --repo "https://github.com/nodejs/bluesky" \
  -F prTitle='vx.x.x release announcement' \
  -F richText='Node.js vx.x.x is out. Check the blog post at https://nodejs.org/…. TL;DR is

- New feature
- …'

# Create a PR for a retweet:
gh workflow run create-pr.yml --repo "https://github.com/nodejs/bluesky" \
  -F prTitle='Retweet vx.x.x release announcement' -F postURL=…

# Create a PR for a quote-RT:
gh workflow run create-pr.yml --repo "https://github.com/nodejs/bluesky" \
  -F prTitle='Retweet vx.x.x release announcement' -F postURL=… \
  -F richText=…