Add action

Add an `action.yml` to make this repository usable as an action.

The usage should look like this

```yml
name: Update discussion toc
on: discussion_comment

permissions:
discussions: write

jobs:
update_toc:
runs-on: ubuntu-latest
if: github.event.discussion.number == 1
steps:
- uses: gr2m/github-discussion-toc@v1.x
```

The action should also accept the optional arguments

- token - defaults to `github.token`
- discussion_url - defaults to `github.event.discussion.html_url`