Add xsd:decimal support to the vocabulary type system

[scammo]

Summary

Add support for the xsd:decimal XML Schema datatype in the vocabulary type system, so properties like hasNumericalValue can use a semantically correct, precision-safe type.
Context: PR #579 review discussion.

Problem

The vocabulary type system currently has no support for xsd:decimal. In PR #579 (FEP-0837 Federated Marketplace vocabulary), the Measure type's hasNumericalValue property had to use xsd:string as a stopgap because xsd:float introduces IEEE 754 floating-point rounding errors (e.g., 0.1 + 0.2 !== 0.3), which are hardly acceptable in monetary and marketplace contexts.

Using xsd:string works but loses type-level semantics and requires manual parsing and validation by consumers.
More context at the following issues "Add Note about xsd:float and xsd:double Limitation"

Proposed Solution

Add xsd:decimal as a supported range type in the vocabulary type system. Once supported, properties like hasNumericalValue can be updated from xsd:string to xsd:decimal.

The key design decision is how to represent xsd:decimal values in TypeScript. This should be discussed before implementation.

Alternatives Considered

• number: Simple, but subject to floating-point precision issues. Unsuitable for financial values.
• string: Safe and dependency-free, but loses type-level semantics and requires manual parsing/validation. This is the current stopgap used in PR Add vocabulary relating to proposals, intentions and measures.  #579.
• A Decimal library (e.g., decimal.js): Provides arbitrary-precision arithmetic, but adds a runtime dependency.

Scope / Dependencies

• @fedify/vocab-tools: Code generation would need to handle xsd:decimal as a range type.
• @fedify/vocab: Vocabulary YAML files using xsd:string as a workaround (e.g., measure.yaml) could be updated once support lands.