Automatically post checklist comment for new components

When a PR includes a new folder in `extensions/`, @ampprojectbot will post a checklist as a comment:

> It looks like you're submitting a new extension. 👁️📎👁️
>
> Please make sure you're following the [contribution guidelines](https://go.amp.dev/contribute/code). These steps need to be completed before code review:
>
> - [ ] This extension has an [intent-to-implement issue (I2I)](https://go.amp.dev/i2i) that is attached to this pull request. It documents:
> - [ ] the **features** the extension provides
> - [ ] the **attributes** the element uses, if any
> - [ ] how attributes are **validated**
> - [ ] how **the page is modified**: _"my component creates an `` that loads `$URL`"_
> - [ ] **if a video player**, it explains purpose according to [video component policy](https://github.com/ampproject/amphtml/blob/master/spec/amp-3p-video.md)
> 
> - [ ] The I2I issue tags a [working group](https://github.com/ampproject/meta/tree/master/working-groups):
> - if the extension is an ad type, or an integration for analytics:
> `@ampproject/wg-ads-and-analytics`
> - otherwise:
> `@ampproject/wg-ui-and-a11y`.
> 
> - [ ] The I2I issue has been approved during [a design review](https://github.com/ampproject/amphtml/blob/master/contributing/design-reviews.md).

It could block `OWNERS` check:

> After you've checked all items in this list, an appropriate reviewer will be automatically assigned.

**or**:

> - [ ] This pull request adds an [`extension/amp-new-extension/OWNERS`](https://github.com/ampproject/amphtml/blob/e33dd10533805655835c196389e4b5ea91f080b1/contributing/CODE_OWNERSHIP.md) file that includes:
> - [ ] yourself (`username`)
> - [ ] the relevant [working group](https://github.com/ampproject/meta/tree/master/working-groups) (like `ampproject/wg-ads-and-analytics`)

/to @ampproject/wg-infra