feat: enhanced custom function registration

[cawalch]

Adds enhanced function registration API with type safety and error handling.

Maintain the existing registerFunction API for backward compatibility.

import { register, search, TYPE_NUMBER } from "@jmespath-community/jmespath";

// TypeScript prevents registering built-in functions at compile time
// register('sum', myFunc, signature); // TypeScript error!

// Enhanced registration with better error handling
const result = register('multiply', ([a, b]) => a * b, [
  { types: [TYPE_NUMBER] },
  { types: [TYPE_NUMBER] }
]);

if (result.success) {
  console.log(result.message); // "Function multiply() registered successfully"
} else {
  console.error(result.message); // Detailed error information
}

import { registerFunction, register } from "@jmespath-community/jmespath";

// Option 1: Using registerFunction with options
registerFunction('myFunc', () => 'first', []);
registerFunction('myFunc', () => 'second', [], { override: true, warn: true });
// Console: "Warning: Overriding existing function: myFunc()"

// Option 2: Using enhanced register API
const result = register('myFunc', () => 'third', [], { override: true });
console.log(result.message); // "Function myFunc() overridden successfully"

import {
  isRegistered,
  getRegisteredFunctions,
  getCustomFunctions,
  unregisterFunction,
  clearCustomFunctions
} from "@jmespath-community/jmespath";

// Check if function exists
console.log(isRegistered('sum')); // true (built-in)
console.log(isRegistered('myFunc')); // true (if registered)

// Get all registered functions
const allFunctions = getRegisteredFunctions();
console.log(allFunctions); // ['abs', 'avg', 'ceil', ..., 'myFunc']

// Get only custom functions
const customFunctions = getCustomFunctions();
console.log(customFunctions); // ['myFunc', 'divide', ...]

// Unregister custom function (built-ins cannot be unregistered)
const removed = unregisterFunction('myFunc');
console.log(removed); // true if successful

// Clear all custom functions
clearCustomFunctions();
console.log(getCustomFunctions()); // []

[springcomp]

Thanks, that is an awesome improvement.