WIP: [ffigen] Separate Symbols into distinct kinds

[liamappelbe]

Fixes #2054

[github-actions]

PR Health

Breaking changes ✔️

Package	Change	Current Version	New Version	Needed Version	Looking good?
objective_c	Breaking	8.1.0	9.0.0-dev.0	9.0.0	✔️

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Changelog Entry ❗

Package	Changed Files
package:objective_c	pkgs/objective_c/lib/src/c_bindings_generated.dart pkgs/objective_c/lib/src/objective_c_bindings_generated.dart pkgs/objective_c/lib/src/selector.dart

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

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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 symbol	Leaking sources
objective_c	_FinalizablePointer	internal.dart::_ObjCReference::new::_finalizable

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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/objective_c/lib/src/ns_input_stream.dart

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[coveralls]

coverage: 80.804%. remained the same
when pulling 9838f7e on ffigen_keyword
into 0924cb0 on main.