[native_assets_cli] Separate out json_syntax_generator into separate package

[dcharkes]

Bug: #1826

This refactors package:hooks tool/generate_syntax.dart.

• Moved into its own package package:json_syntax_generator.
• Documented the feature set: optional/required fields, subclassing, tagged unions, and enums.
• Refactored from a single-pass code generator, to a two step pipeline. (1) analyze the JSON schema and extra into a data structure, (2) generate code from the data structure.
• Sprinkled comments about the "why" on the most important classes and fields.

This PR is intended to be a NOP w.r.t. the syntax that's generated. (Some whitespace and toString changes.)

Having this as a separate package will enable us to reuse it for the syntax in different places. For now, we're not intending to publish it, instead we can use it as a git dependency in the dev dependencies (or we publish it unlisted). @mosuem Please let me know if it is possible for you to generate the syntax for the FontAssets.

Open issue:

• The generated code requires a utils/json.dart. We should make the generated code be standalone. I'll do that in a follow up PR.

[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