feat(json): add reusable JSON serialization options

[GaspardKirira]

Summary

Provide one options structure for string serialization and file output.

Current behavior

Serialization behavior is configured through positional parameters such as:

• indentation;
• ASCII escaping.

As additional options are added, positional overloads become harder to read and extend.

Expected behavior

The module should provide an explicit options type while keeping existing overloads.

Scope

• Define a serialization options structure.
• Support compact output.
• Support pretty output.
• Support indentation width.
• Support ASCII escaping.
• Define invalid indentation behavior.
• Integrate options with dumps().
• Integrate options with dump_file().
• Preserve current overloads.
• Add tests.

Suggested files

• modules/json/include/vix/json/dumps.hpp
• modules/json/tests/json_dumps_options_test.cpp

Acceptance criteria

• Existing positional overloads remain compatible.
• Options-based compact output works.
• Options-based pretty output works.
• ASCII escaping follows the selected option.
• Invalid option combinations fail clearly.
• File and string serialization use the same option semantics.

Non-goals

This issue does not include:

• custom JSON encoders;
• streaming output;
• schema formatting;
• canonical JSON.