Add Version 1.5.0 changelog to README

[Copilot]

The README changelog was missing the 1.5.0 entry. This adds a ## Version 1.5.0 section documenting all significant changes since the xsd-parser-v1.4.0 tag, compiled from merged PRs and related issues.

Changes documented

• xs:anySimpleType support — new AnySimpleType runtime type in xsd-parser-types; configurable via generator
• xs:redefine / xs:override support — schema composition mechanisms now parsed and interpreted correctly
• Namespace-aware enum serialization/deserialization — enum values with namespace-prefixed content (e.g. tns:Sender) now resolved by URI rather than literal prefix string matching
• Duplicated type identifiers across schema files — Ident split into TypeIdent (with SchemaId), NodeIdent, PropertyIdent to handle schemas reusing type names across files
• DerivedTypeMeta — tracks derived type display names in DynamicMeta to resolve cross-schema naming conflicts
• Non-zero integer types — XSD integer constraints can now map to Rust NonZero* types independent of WITH_NUM_BIG_INT
• Improved name generation — Naming trait handles duplicate group names and nested type collisions
• Anchored regex pattern validation — xs:pattern facets now correctly anchored (^(?:...)$) per XSD spec
• Improved quick_xml deserialization — dynamic types, mixed content, deeply nested structures
• New examples and benchmarks — BPMN 2.0 and VSME example projects; memory and performance benchmark crate
• Bug fixes — list deserialization edge cases, namespace serialization for Element, non-whitespace list separators, attribute character escaping, group type naming collisions, WithNamespaceTrait conflicting impls for type aliases, empty list error handling, with_element_type_postfix typo

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