parser: add speculative page IDs to use with {{PAGEID}}

parser: add speculative page IDs to use with {{PAGEID}}

This works similarly to speculative rev IDs with {{REVISIONID}}.
Re-parses can be avoided if the page ID is correctly guessed.

Also make the {{PAGEID:X}} parser function set vary-page-id.

Bug: T226785
Change-Id: I0b19be45e6ddd6cde330bfcd09d243e4e5beda01