[Test] C-API

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RBS C Parser Library Refactoring

This PR refactors the RBS parser and related components into a standalone C library that no longer depends on the Ruby runtime. This architectural change enables direct integration with static analysis tools like Sorbet while potentially improving performance.

Sorbet's RBS support already runs on this new architecture and we haven't discovered any major issues around it.

Key Improvements

• Ruby-Independent Implementation: Extracted parser from ext folder into a standalone C library with a clean API, which can now be embedded in non-Ruby tools without Ruby runtime dependency (e.g. Sorbet, JRuby)
• Enhanced Memory Management: Implemented arena allocator to efficiently manage parser object lifecycles
• Improved Architecture: Clear separation between public API (headers) and implementation
• Performance: Potential performance gains from custom memory management and reduced overhead

Enhanced Memory Management

Arena allocator handles all memory for parser objects, including parser itself, lexer, constant pool, strings...etc. When the parser is freed by calling rbs_parser_free, the allocator will free all the objects it allocated. This eliminates the need to manually free individual objects and reduces the risk of memory leaks.

Component Architecture

graph TD
    RubyClient[Ruby Client] --> RubyAPI[Ruby API]
    CClient[C Client] --> CAPI[C API]

    RubyAPI --> CExtension[C Extension]
    CExtension --> CLibrary
    CAPI --> CLibrary

    subgraph CLibrary[C Library]
        subgraph Parser1[Parser Instance 1]
            direction TB
            ConstantPool1[Constant Pool]
            Lexer1[Lexer]
            ArenaAllocator1[Arena Allocator]
        end

        subgraph Parser2[Parser Instance 2]
            direction TB
            ConstantPool2[Constant Pool]
            Lexer2[Lexer]
            ArenaAllocator2[Arena Allocator]
        end
    end

    subgraph "Public API"
        RubyAPI
        CAPI
    end

    %% Parser1 --> ConstantPool1
    %% Parser1 --> Lexer1
    %% Parser1 --> ArenaAllocator1
    %% Parser2 --> ConstantPool2
    %% Parser2 --> Lexer2
    %% Parser2 --> ArenaAllocator2

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I'll leave this PR open because ruby/rbs only runs CI on pull requests, so we need this PR to have CI constantly running against changes in c-api.