Convert to Reference Link code action

[bennypowers]

Thank you for publishing this useful package

given this file:

This paragraph has [a link](https://a.website) inline.

Let there be a code action called "Convert to Reference Link" which converts the file to

This paragraph has [a link][a-link] inline.

[a-link]: https://a.website

The code action would know to sort existing reference links by first-appearance, and would offer the user an input to choose the reference tag, pre-populated with the slugified link text.