[ffigen] Replace Writer with Context in all Type to string methods

[liamappelbe]

This is purely a plumbing change.

Fixes #2462
Yak shaving for #2419

[github-actions]

PR Health

Breaking changes ✔️

Package	Change	Current Version	New Version	Needed Version	Looking good?
jni	Breaking	0.14.2	0.15.0-wip	0.15.0	✔️

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// 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.

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pkgs/objective_c/lib/src/ns_input_stream.dart

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[coveralls]

coverage: 81.314% (+0.4%) from 80.915%
when pulling 6091ce7 on ffigen_typewriter
into 76d519a on main.

[HosseinYousefi]

Is this still a draft or is it ready to review?

[HosseinYousefi]

The change itself seems trivial so LGTM nevertheless

[liamappelbe]

Is this still a draft or is it ready to review?

Oops, forgot to mark it ready