feat(scanner): extract arrow functions and function expressions

[prosdev]

Summary

Implements arrow function and function expression extraction for the TypeScript scanner, addressing issue #119.

Changes

New Features

• Variable extraction: Extracts arrow functions and function expressions assigned to const/let variables
• Hook detection: Automatically detects React hooks by use* naming convention
• Async detection: Flags async arrow functions via isAsync metadata
• Full metadata: Extracts callees, JSDoc, imports, and signatures from function bodies

API Additions

• Added 'variable' to DocumentType union type
• Added optional metadata fields:
  • isArrowFunction: boolean - true for arrow functions (vs function expressions)
  • isHook: boolean - true if name matches /^use[A-Z]/
  • isAsync: boolean - true for async functions

Examples

Now extracts:

export const useAuth = () => { ... }           // Hook
export const fetchData = async (url) => { ... } // Async function
const validateEmail = (email: string) => ...   // Utility function
const legacy = function(a, b) { return a + b } // Function expression

Still skips (as intended):

const API_URL = '/api'           // Primitive constant
const config = { timeout: 5000 } // Object literal

Testing

• Added 11 new tests covering all extraction patterns
• All 1483 tests pass
• Lint and typecheck pass

Closes

Closes #119