[NumberFormatter] Implement `signDisplay` `negative` option polyfill

### Provide a general summary of the feature here

### 🤔 Expected Behavior?

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### 😯 Current Behavior

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### 💁 Possible Solution

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### 🔦 Context

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### 💻 Examples

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### 🧢 Your Company/Team

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### 🕷 Tracking Issue

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The issue names NumberFormatter and the signDisplay negative option; first locate the NumberFormatter implementation and its existing signDisplay tests or handling. Confirm the intended negative-sign behavior and add coverage showing the option works in the polyfill.

Written by the indexing model from the issue text.