Summary:
- Anchor links with row of 'new' could be created but weren't parsed or used
correctly. This fixes it.
- Also adds UIRowId type for row IDs which includes the special 'new' row. It's
already been used in places as `number|'new'`, this diff gives it a name usable in app/common
(it doesn't touch another name, RowId, that's been available in app/client).
Test Plan: Added a test assert for anchor links to new row
Reviewers: alexmojaki
Reviewed By: alexmojaki
Differential Revision: https://phab.getgrist.com/D3039
