Reimplement Varnish cache purging

[jaymcp]

The default implementation assumes:

• the home page is the blog index
• one doesn't make use of attachment pages
• only posts and pages exist
• only category and tag taxonomies exist
• the base path of the REST API uses permalinks

This version reimplements path generation to
address what in my opinion are shortcomings.

Ref: https://github.com/amnestywebsite/amnesty-wp-theme/issues/3101

Steps to test:

1. testing requires a production environment with page caching enabled, but edge page caching disabled. liaise with @jaymcp to organise one.
2. open an incognito window, and open dev tools to the network tab. filter requests by "doc"
3. view a post/page/petition/etc.
4. inspect the page view request, and ensure that the cache header shows a HIT (reload if not)
5. make & save an edit to the item in your main browser window, not the incognito one
6. reload the incognito window
7. cache header should show a MISS, and the updated content should show
8. reload the incognito window
9. cache header should show a HIT
10. delete the item from your main browser window
11. reload the incognito window
12. the item should immediately 404
13. still in the incognito window with devtools open, view a document page
14. repeat steps 4-12 against this document