Add UBL parser for Invoice and CreditNote documents

[Copilot]

• Explore repository structure and understand existing code
• Create src/parser/parserOptions.ts – XMLParser options (mirroring builder options)
• Create src/parser/DocumentParser.ts – parser class with parseInvoice() and parseCreditNote() methods
• Create src/parser/index.ts – barrel export for parser module
• Update src/index.ts – add peppolUBLToInvoice() and peppolUBLToCreditNote() to PeppolToolkit
• Create tests/parser-invoice.test.ts – 19 round-trip tests for Invoice parsing
• Create tests/parser-creditnote.test.ts – 18 round-trip tests for CreditNote parsing
• Merge main into branch
• Bump package version to 1.0.0

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[SimonLoir]

@copilot merge main