Cannot find module '.../generate-slug' imported from .../anchor-links/index.js

# Versions

- @hashicorp/platform-remark-plugins@0.2.0
- Node: v14.18.2

# Summary

I just tried upgrading from the older `@hashicorp/remark-plugins` to `@hashicorp/platform-remark-plugins`. I immediately get an error when trying to import `anchorLinks`:

```
Error [ERR_MODULE_NOT_FOUND]: Cannot find module '/home/mark/xxx/node_modules/@hashicorp/platform-remark-plugins/util/generate-slug' imported from /home/mark/xxx/node_modules/@hashicorp/platform-remark-plugins/plugins/anchor-links/index.js
at new NodeError (internal/errors.js:322:7)
at finalizeResolution (internal/modules/esm/resolve.js:308:11)
at moduleResolve (internal/modules/esm/resolve.js:731:10)
at Loader.defaultResolve [as _resolve] (internal/modules/esm/resolve.js:842:11)
at Loader.resolve (internal/modules/esm/loader.js:89:40)
at Loader.getModuleJob (internal/modules/esm/loader.js:242:28)
at ModuleWrap. (internal/modules/esm/module_job.js:76:40)
at link (internal/modules/esm/module_job.js:75:36) {
code: 'ERR_MODULE_NOT_FOUND'
}
```

# Analysis

I believe the problem here is that the package has been migrated to ESM incompletely. Specifically, imports in an ESM project must include the file extension. This seems to have been done for most files, but `packages/remark-plugins/plugins/anchor-links/index.js` still has an import without a file extension. I believe line 1 of that file needs to change:

```diff
- import { generateSlug, generateAriaLabel } from '../../util/generate-slug'
+ import { generateSlug, generateAriaLabel } from '../../util/generate-slug.js'
```