[ffigen] Fix usrTypeMappings field of Config factory constructor

[liamappelbe]

Unlike typedefTypeMappings, structTypeMappings, unionTypeMappings, and nativeTypeMappings, usrTypeMappings is keyed by the USR, which is not a field on ImportedType. So usrTypeMappings can't be constructed from a List<ImportedType>, and we have to take the Map<String, ImportedType> as an input instead.

Also, add the changelog and readme entries that I forgot to add in #2007

Fixes #1999

[github-actions]

PR Health

Breaking changes ✔️

Package	Change	Current Version	New Version	Needed Version	Looking good?

Changelog Entry ✔️

Package	Changed Files

Changes to files need to be accounted for in their respective changelogs.

API leaks ✔️

The following packages contain symbols visible in the public API, but not exported by the library. Export these symbols or remove them from your publicly visible API.

Package	Leaked API symbols

License Headers ✔️

// Copyright (c) 2025, the Dart project authors. Please see the AUTHORS file
// for details. All rights reserved. Use of this source code is governed by a
// BSD-style license that can be found in the LICENSE file.

Files
no missing headers

All source files should start with a license header.

Unrelated files missing license headers

Files
pkgs/jni/lib/src/third_party/generated_bindings.dart
pkgs/objective_c/lib/src/ns_input_stream.dart

[coveralls]

coverage: 87.64% (+0.004%) from 87.636%
when pulling b80a8b3 on configapifix
into a74e5a2 on main.