feat(types): improve isArray option type inference

[abhu85]

Summary

This PR improves the TypeScript type definitions for the isArray option and other callbacks that receive jPath or Matcher parameters, addressing the concerns raised in #803.

The Problem

The current type definition uses string | Matcher (where Matcher = unknown) for callbacks like isArray, tagValueProcessor, etc. This forces users to use type assertions even when they know jPath: true (the default):

// Current behavior - TypeScript error!
const options: X2jOptions = {
  isArray: (_tagName, jPath, _isLeafNode, _isAttribute) => {
    // Error: Argument of type 'string | unknown' is not assignable to parameter of type 'string'
    return ['root.items'].includes(jPath);
  }
};

The Solution

This PR introduces discriminated union types based on the jPath option:

// Now works without type assertions!
const options: X2jOptions = {
  // jPath defaults to true, so jPath is inferred as string
  isArray: (_tagName, jPath, _isLeafNode, _isAttribute) => {
    return ['root.items'].includes(jPath); // jPath is string
  }
};

// Explicit jPath: true
const optionsWithString: X2jOptions = {
  jPath: true,
  isArray: (_tagName, jPath, _isLeafNode, _isAttribute) => {
    return jPath.startsWith('root.'); // jPath is string
  }
};

// Explicit jPath: false
const optionsWithMatcher: X2jOptions = {
  jPath: false,
  isArray: (_tagName, matcher, _isLeafNode, _isAttribute) => {
    // matcher is Matcher (unknown) - users must handle appropriately
    return false;
  }
};

New Exported Types

• X2jOptionsBase: Base options shared by both jPath modes
• X2jOptionsWithJPathString: Options when jPath: true (callbacks receive string)
• X2jOptionsWithMatcher: Options when jPath: false (callbacks receive Matcher)
• X2jOptions: Union of the above two types

Affected Callbacks

All callbacks that previously used string | Matcher now properly narrow based on jPath:

• isArray
• tagValueProcessor
• attributeValueProcessor
• updateTag
• ignoreAttributes (function form)

Test Plan

• TypeScript compilation passes for the type definition file
• Added type test files demonstrating correct type inference
• Added type error test files verifying TypeScript catches incorrect usage
• Existing test suite passes (307 specs, 1 unrelated failure)

Fixes #803