This PR is part 3 of addressing #609, and it is stacked on top of #1980 and #1981 (and therefore contains also the commits of those PRs), therefore I will leave this in draft mode until those PRs are merged.

The grand idea is to bundle the `CIHelper` class together with all its direct and transitive dependencies into one big, honking `dist/index.js`, and then add a set of really minimal GitHub Actions that call into `CIHelper`. The Actions are added in sub-directories so that they can be called in GitHub workflows via e.g. `- uses: gitgitgadget/gitgitgadget/update-prs@1`.

The component used for bundling `CIHelper` is [`@vercel/ncc` ](https://www.npmjs.com/package/@vercel/ncc). To support acting as a GitHub Action, [`@actions/core`](https://www.npmjs.com/package/@actions/core) is installed.

To allow for really minimal GitHub Actions, the `CIHelper` class is augmented accordingly to re-implement more logic that is currently either in `misc-helper.ts` or in the (non-public 😞) Azure Pipelines definitions.

The naming convention for specifying the necessary tokens as GitHub Actions inputs is:
- `upstream-repo-token`: This is to comment on PRs in `git/git`
- `pr-repo-token`: This is to comment on PRs in `gitgitgadget/git` (as well as to be able to push to that repository)
- `test-repo-token`: This is to comment on PRs in `dscho/git` (used exclusively for testing)

To clarify, here is a diagram:

```mermaid
graph TD
    user["user (contributor)"]
    upstream-repo["upstream-repo (authoritative project repo)"]
    pr-repo["pr-repo (GitGitGadget-enabled GitHub repo)"]
    GitGitGadget["GitGitGadget"]
    mailing-list["mailing-list"]

    user -->|"opens PR"| pr-repo
    user -->|"opens PR (if GitHub App installed)"| upstream-repo

    upstream-repo -->|"GitGitGadget syncs branches to"| pr-repo

    pr-repo -->|"slash commands"| GitGitGadget
    upstream-repo -->|"slash commands (if App installed)"| GitGitGadget

    GitGitGadget -->|"sends patch series"| mailing-list
```

Publish the GitHub Actions to a separate branch

This PR is part 4 (the final part in this repository) of addressing #609, and it is stacked on top of #1980, #1981, and #1982 (and therefore contains also the commits of those PRs), therefore I will leave this in draft mode until those PRs are merged.

After laying the groundwork for, and implementing, the set of GitHub Actions that can perform the same job as GitGitGadget's current Azure Pipelines can perform, this here PR adds automation to
1. transpile the `CIHelper` class (together with its dependencies) from Typescript to JavaScript,
2. bundle it as a single, dependency-less `dist/index.js`,
3. copy the required resources (`WELCOME.md`, some shell scripts) into the location expected by that `dist/index.js`,
4. remove all the rest except for the minimal GitHub Actions (`*/action.yml`, `*/index.js`),
5. commit the result as the new tip commit of the `v1` branch (creating it as needed),
6. tag that tip commit as `v1.<running-number>`,
7. push out the `v1` branch and the tag.

The result of this is that GitGitGadget can still be developed conveniently in this here repository, and whenever anything gets merged to the `main` branch, the `v1` branch is automatically updated so that it will be picked up by GitHub workflows containing statements like:

```yaml
- uses: gitgitgadget/gitgitgadget/handle-pr-comment@v1
```

That way, we _finally_ address the fragile nature of the current setup where a set of Azure Pipelines maintain their own clone of `gitgitgadget/gitgitgadget`, and having to run `npm ci && npm run build` as needed.

This closes #1759