Add configurable Producer metadata to address information disclosure (#3878)

[JeelGajera]

Add configurable Producer metadata property

Allow users to override the PDF Producer field via setDocumentProperties() and setDocumentProperty() to address information disclosure concerns.

Usage:

// Set custom producer
const doc = new jsPDF();
doc.setProperties({ producer: 'My Custom App v1.0' });

// Or via setDocumentProperty
doc.setDocumentProperty('producer', 'Custom Producer');

// Default behavior (unchanged) omit producer to get "jsPDF <version>"
const doc2 = new jsPDF();

Fixes #3878