feat(fixer_v2): State Transition Machine for Metavariable Parser

[notJoon]

Description

stateDiagram-v2
    direction LR
    
    GO --> CL: "COLON"
    CL --> OB: "["
    OB --> DB: "["
    OB --> NM: "ident"
    DB --> NM: "ident"
    
    %% Simple Pattern :[name]
    NM --> NM: "ident"
    NM --> CB: "]"
    CB --> OK: "COLON" or "whitespace"
    CB --> QB: "]"
    CB --> QT: "*+?"
    
    %% Type Pattern :[[name:type]]
    NM --> ID: "COLON"
    ID --> ID: "ident"
    ID --> CB: "]"
    
    %% Post Quantifier
    QB --> QT: "*+?"
    QB --> OK: "COLON" or "whitespace"
    QT --> OK: "COLON" or "whitespace"
    
    %% Text handling
    GO --> TX: "other"
    GO --> WS: "whitespace"
    GO --> BR: "{}"
    
    %% Whitespace loop
    WS --> WS: "whitespace"
    WS --> CL: "COLON"
    WS --> TX: "other"
    WS --> BR: "{}"
    
    %% Block handling
    BR --> BR: "{"
    BR --> OK: "}"
    BR --> CL: "COLON"
    BR --> WS: "whitespace"
    
    note right of GO
        Initial state
    end note
    
    note right of CL
        After colon
    end note
    
    note right of NM
        Reading name
    end note
    
    note right of ID
        Reading type
    end note
    
    note right of QT
        Reading quantifier
    end note

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The initial implemtation of state transition machine, which will replaces the current naive lexer implementation. This is a drop-in replacement for the existing lexer. No changes required in the parser or other components for now.