[infra] Create a pub workspace

[dcharkes]

Bug:

• [infra] Convert this mono_repo to use pub workspaces #1223

Split off:

• [infra] Use Pub Workspaces #1884

🙏 @Levi-Lesches !

Included packages:

• pkgs/code_assets
• pkgs/data_assets
• pkgs/hooks
• pkgs/json_syntax_generator
• pkgs/native_assets_builder
• pkgs/native_assets_cli
• pkgs/native_toolchain_c

These packages are developed in lock-step, and might want a dev release as lower bound. (FFIgen, JNIgen, etc. probably want to always have a stable SDK as lower bound.)

This PR is the minimal PR to enable pub workspaces. It enables single package resolution in the workspace, but does not clean anything up: the path dependencies are not removed yet. We should be able to remove these and simply refer to the WIP versions.

[github-actions]

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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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