[pigeon] add support for top level consts

[tarrinneal]

Add Top-level Constants Support to Pigeon

This PR adds support for defining and generating top-level constants across all target languages supported by Pigeon (Dart, Kotlin, Java, Swift, Objective-C, C++, and GObject).

Resolves: flutter/flutter#188642

Summary of Changes

1. AST & Parser

• Added a Constant node type to the AST representing top-level constants.
• Updated pigeon_lib_internal.dart to parse const declarations.
• Restrained constant types to: String, int, double, and bool.
• Implemented validation for explicit type annotations, type matching, and initialization values (unsupported types, binary expressions, and string interpolations will trigger compilation errors).

2. Code Generation

Added constant generation to all target languages:

• Dart: Generates top-level const variables.
• Java: Generates public static final fields inside the main generated class.
• Kotlin: Generates companion object const val properties (using val for double/boolean where JVM const is not supported).
• Swift: Generates global public let constants.
• Objective-C: Generates global static const / static NSString *const variables in the header file.
• C++: Generates global inline constexpr variables in the header.
• GObject: Generates global #define macros in the header.

3. Tests & Documentation

• Added parsing validation tests in test/pigeon_lib_test.dart.
• Added generator unit tests (test/*_generator_test.dart) for each language verifying the syntax of generated constants.
• Updated pigeons/core_tests.dart with constant declarations, and added corresponding unit tests verifying their correctness in each target platform project.
• Added an integration test in platform_tests/shared_test_plugin_code/lib/integration_tests.dart to assert constant values at runtime.
• Updated packages/pigeon/README.md to document the new feature, using code excerpts linked to example/app/pigeons/messages.dart.

[gemini-code-assist]

Code Review

This pull request adds support for generating top-level constants across all supported platforms in Pigeon. Feedback on the changes suggests resolving compilation issues by avoiding the escaping of the $ character in the shared double-quote escaping helper and instead handling it specifically in Kotlin. Additionally, double constants initialized with integer literals should be explicitly parsed as doubles to prevent type mismatch errors in Java and Kotlin, and Java constants should be generated as primitive types rather than boxed types to ensure they function as true compile-time constants.

[tarrinneal]

/gemini

[tarrinneal]

/gemini