tpl/collections: Honor the Eqer interface in where comparisons

tpl/collections: Honor the Eqer interface in where comparisons

The where function previously fell through to a no-op when comparing
two values whose kinds were not handled by the primitive type switches
(e.g. two Page interface values). This made `where pages "Parent" $page`
return an empty list, while the equivalent `range pages` + `if eq` worked.

Use compare.Eqer for equality operators when either side implements it,
matching the behavior of the eq/ne template funcs.

Fixes #14777

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>