Publish `openapi-diff` Using ERSP

[scbedd]

@konrad-jamrozik the consequence of this PR is that you will be able to release oad through an approvable step on the internal build.

That will probably mean creating an internal version of the existing build, but nbd.

Related to Azure/azure-sdk-tools#7941

[konrad-jamrozik]

@scbedd so I understand the beginning of these instructions will change:

https://github.com/Azure/openapi-diff/blob/main/CONTRIBUTING.md#publish-the-package

Instead of:

- Ensure you bumped the package version in `openapi-diff` `package.json`.
- Verify [`public.openapi-diff`] passed.
- Use [`js - tools to npm - publish (@azure)`] to publish the package to the public `npm` feed.
- Verify in [`@azure/oad` versions] the package was published.

It will be something like:

- Ensure you bumped the package version in `openapi-diff` `package.json`.
- Verify [`internal.openapi-diff`] passed. <------------- note the change from `public` to `internal` that needs to be created
- Verify in [`@azure/oad` versions] the package was published.

[scbedd]

@scbedd so I understand the beginning of these instructions will change:

https://github.com/Azure/openapi-diff/blob/main/CONTRIBUTING.md#publish-the-package

Instead of:

- Ensure you bumped the package version in `openapi-diff` `package.json`.
- Verify [`public.openapi-diff`] passed.
- Use [`js - tools to npm - publish (@azure)`] to publish the package to the public `npm` feed.
- Verify in [`@azure/oad` versions] the package was published.

It will be something like:

- Ensure you bumped the package version in `openapi-diff` `package.json`.
- Verify [`internal.openapi-diff`] passed. <------------- note the change from `public` to `internal` that needs to be created
- Verify in [`@azure/oad` versions] the package was published.

Basically! I'll update the contributing guide.