feat: add SingularNullable getter arity and singular error handling

[Copilot]

Adds more arities for selecting terms beyond Singular, and adds proper error handling to singular() per the approach in #13.

Changes

• GetterArity.SingularNullable — new enum variant enabling nullable term selection via decorators, complementing the existing singularNullable() method on TermWrapper
• getter decorator — routes SingularNullable to this.singularNullable()
• singular() error handling — throws descriptive error when no matching quad exists instead of crashing on undefined. Uses iterator to avoid materializing all matching quads (relevant for lazy datasets like N3.js)

Usage

// Decorator-based nullable getter (new)
@getter(Vocabulary.hasName, GetterArity.SingularNullable, ValueMappings.literalToString)
public get hasName(): string | undefined {
    throw new Error
}

// singular() now throws with a clear message when value is missing
// "No value found for predicate https://example.org/hasChild on term y"

Original prompt

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This section details on the original issue you should resolve

<issue_title>Improvement: More arities</issue_title>
<issue_description></issue_description>

Comments on the Issue (you are @copilot in this section)

@jeswr As I understand, this PR should implement more arities for selecting terms; in addition to `signular`. This should contain the same kind of error handling logic that exists in https://github.com//pull/13

• Fixes Improvement: More arities #8

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[langsamu]

The parts related to new branch on decorator are fine.
Others are unrelated and should be removed.

[langsamu]

Changes here are unrelated and should be removed.
NB covered by #13.

[langsamu]

As per https://github.com/theodi/wrapper/pull/18/changes#r2800076109

[langsamu]

This change does not fix #8, which is a stub issue for the much broader, generic concern of adding many more 'arities' (a misnomer while we come up with better terminology).