Add simple PR comment sub-action (#636)

* Add simple PR comment sub-action

* Update rolldown config

* Add changeset

* Update readme

* Use TLA

* Add prefix

* fix merge

* Update

pr-comment/README.md

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+# @changesets/action/pr-comment
+
+A simple GitHub Action to comment on PRs aimed to complement [`@changesets/action/pr-status`](../pr-status/README.md).
+
+This action is intentionally simple without advanced features. Check out other actions if so, such as [mshick/add-pr-comment](https://github.com/marketplace/actions/add-pr-comment) and [peter-evans/create-or-update-comment](https://github.com/marketplace/actions/create-or-update-comment).
+
+See the [action metadata](action.yml) for details on the inputs and outputs.
+
+## Example setup
+
+```yaml
+name: PR Comment
+
+on:
+  pull_request:
+
+jobs:
+  pr-comment:
+    runs-on: ubuntu-slim
+    permissions:
+      pull-requests: write # to create and update comments on PRs
+    steps:
+      - uses: changesets/action/pr-comment@v1
+        with:
+          body: Hello world!
+```
+
+When called repeatedly, the action will update the comment it created by default. If you use this action to create different types of comments, pass an `update-id` value to differentiate them.
+
+```yaml
+jobs:
+  pr-comment:
+    # ...
+    steps:
+      - uses: changesets/action/pr-comment@v1
+        with:
+          body: Hello world!
+          update-id: my-tag
+```
+
+If you want to always create new comments, pass an empty value to `update-id`.
+
+```yaml
+jobs:
+  pr-comment:
+    # ...
+    steps:
+      - uses: changesets/action/pr-comment@v1
+        with:
+          body: Hello world!
+          update-id: ""
+```

pr-comment/action.yml

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+name: Changesets - PR Comment
+description: A simple GitHub Action to comment on PRs
+runs:
+  using: node24
+  main: ../dist/pr-comment.js
+inputs:
+  github-token:
+    description: "The GitHub token to use for authentication. Defaults to the GitHub-provided token."
+    required: false
+    default: ${{ github.token }}
+  body:
+    description: "The comment body to post on the PR."
+    required: true
+  update-id:
+    description: >
+      By default, the action will create and update a comment with this id. Pass a different id to
+      create and update a new comment, or pass an empty string to disable updating comments.
+    required: false
+    default: "changesets-action-pr-comment"
+outputs:
+  comment-id:
+    description: "The comment id of the comment that was created or updated."
+branding:
+  icon: message-circle
+  color: blue