[native_assets_cli] Make generated syntax files stand on their own

[dcharkes]

Bug: #1826

package:json_syntax_generator is only reusable if the files are self contained.

Alternatively, we could make the generator take a config option for a JSON import. However, the downside would be that we would need copy paste the library file to multiple places.

Additionally, this PR cleans up all other uses of the utils/json.dart in package:native_assets_cli. The only remaining JSON helper is sortOnKey() because the asset-extension touches JSON manually and needs to sort the keys. (This could be addressed when addressing #2090.)

[github-actions]

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Breaking changes ✔️

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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.

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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.

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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: 86.587% (-0.04%) from 86.63%
when pulling b7d0007 on refactor-generator-2
into a3aad77 on main.