Add toMapNotNull operator and fix mapNotNull to filter undefined

[iliagon]

Summary

This PR adds a new toMapNotNull operator and fixes mapNotNull to filter out both null and undefined values.

Changes

New Feature: toMapNotNull

• Added toMapNotNull operator for both sync and async sequences
• Filters out key-value pairs where either the key or value is null or undefined
• Returns Map<NonNullable<K>, NonNullable<V>> for proper type safety
• Supports optional map parameter to append to existing map

Fix: mapNotNull

• Updated mapNotNull to filter out both null and undefined values (previously only filtered null)
• Updated return type to Sequence<NonNullable<R>> / AsyncSequence<NonNullable<R>> for better type safety
• Updated documentation to reflect the change

Example Usage

// toMapNotNull example
const map = asSequence([["a", 1], ["b", null], ["c", 2]])
    .toMapNotNull(); // Returns Map with only ["a", 1] and ["c", 2]

// mapNotNull example (now filters undefined too)
const result = sequenceOf(1, 2, 3)
    .mapNotNull(x => x === 2 ? undefined : x)
    .toArray(); // Returns [1, 3]