Add spelling guidelines to CONTRIBUTING.md for American English consistency

[Copilot]

Contributors have been using British spelling in code and documentation, but the TypeScript project uses American English consistently. This PR adds a comprehensive "Spelling Guidelines" section to CONTRIBUTING.md to help contributors maintain consistency.

Changes Made

• Added new "Spelling Guidelines" section after the "Using local builds" section
• Included comprehensive table of common British vs American spelling differences relevant to programming
• Provided TypeScript code examples showing correct and incorrect usage
• Added context-specific examples like backgroundColor, initializeComponent, etc.
• Positioned the section appropriately within the contribution guidelines flow

Examples from the Guidelines

British	American	Examples in Context
behaviour	behavior	defaultBehavior, behaviorSubject
initialise	initialize	initializeComponent, initializeState
analyse	analyze	analyzeCode, dataAnalysis

The section includes practical TypeScript code examples:

// ✅ Correct (American spelling)
interface ComponentBehavior {
    initialize(): void;
    analyze(data: unknown): void;
    synchronizeState(): void;
}

// ❌ Incorrect (British spelling)
interface ComponentBehaviour {
    initialise(): void;
    analyse(data: unknown): void;
    synchroniseState(): void;
}

This will help ensure consistency across all contributions to the TypeScript project.

Fixes #18.

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