feat: make all app unexpanders respond to pp.tagAppFns

[kmill]

This PR changes how app unexpanders are invoked. Before the ref was .missing, but now the ref is the head constant's delaborated syntax. This way, when pp.tagAppFns is true, then tokens in app unexpanders are annotated with the head constant. The consequence is that in docgen, tokens will be linkified. This new behavior is consistent with how notation defines app unexpanders.

In a followup PR we can slightly simplify the notation unexpander macro to not set the ref.

[kmill]

This PR is more general than #6704. If we go with this solution, we can close that PR.

[leanprover-community-bot]

Mathlib CI status (docs):

• ❗ Batteries/Mathlib CI will not be attempted unless your PR branches off the nightly-with-mathlib branch. Try git rebase eb30249b1118b5bea767354d5362f4afba97c82e --onto f9e904af5022a57adb6105cda65d1569449346c2. (2025-01-21 18:11:37)
• ❗ Batteries/Mathlib CI will not be attempted unless your PR branches off the nightly-with-mathlib branch. Try git rebase eb30249b1118b5bea767354d5362f4afba97c82e --onto d68c2ce28bc11ec0271a97aa7a7316905f15a484. (2025-02-02 23:31:30)