FR IdTypeMap: iterate over entries by type

There is already a function [`remove_by_type`](https://docs.rs/egui/0.35.0/egui/util/id_type_map/struct.IdTypeMap.html#method.remove_by_type) to remove all entries of a given type.
I request a function `iter_by_type` to obtain an iterator over all entries of type T.
EDIT: bad link

Start with the IdTypeMap documentation and its existing remove_by_type entry point linked in the issue, then locate the implementation and related tests. Add an iter_by_type API that yields all entries of type T, and verify the iterator behavior with focused tests for matching and non-matching types.

Written by the indexing model from the issue text.