Link validation scripts

### Is there an existing issue for this?

- [x] I have searched the existing issues

### What feature should be added?

Create link validation for internal and external links
Internal: the [remark-validate-links](https://github.com/AdobeDocs/commerce-extensibility/blob/main/package.json#L31) package
* Use [npm script](https://github.com/AdobeDocs/commerce-extensibility/blob/main/package.json#L22) so that our custom [GitHub Action workflow](https://github.com/AdobeDocs/commerce-extensibility/blob/main/.github/workflows/test-pull-request.yml#L82-L83) can run it
External: the [remark-lint-no-dead-urls](https://github.com/AdobeDocs/commerce-extensibility/blob/main/package.json#L30) package

Thanks to @jeff-matthews!

### What is the expected behavior?

Validate links

### How will this feature improve the user experience?

Validate links

### What would a solution for this issue look like?

_No response_

Start with package.json and the referenced .github/workflows/test-pull-request.yml step. Check how the existing npm script is invoked, then review the remark-validate-links and remark-lint-no-dead-urls package references. Done means the workflow runs link validation for both internal and external links.

Written by the indexing model from the issue text.