Add minimal subdomain support

[AdekunleBamz]

This PR adds basic subdomain functionality to the BiUD decentralized username system, allowing users to register subdomains under their existing names.

Changes Made

Contract Updates (contracts/biud-username.clar)

• Added validate-subdomain-label function to handle subdomain validation
• Modified register-name to support subdomain registration with format sub.parent
• Subdomains require the parent domain to be owned by the registrant and not expired
• Subdomains use the same registration fees as regular names

Test Updates (tests/biud-username_test.ts)

• Added comprehensive test suite for subdomain functionality
• Tests cover successful registration, ownership validation, premium status, renewal, transfer, and format validation

Features

• Subdomain Registration: Register names like sub.alice.sBTC under alice.sBTC
• Ownership Validation: Only the parent domain owner can register subdomains
• Standard Fees: Subdomains use the same fee structure as regular names
• Full Lifecycle Support: Subdomains support renewal, transfer, and resolver settings like regular names

Technical Details

• Labels containing one dot are treated as subdomains
• Parent domain must exist and be owned by the registrant
• Full name format: subdomain.parent.sBTC
• Backwards compatible with existing registration flow

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