feat: pin Node.js 20, update contribution docs, add JSDoc examples, a…

[PeterOche]

Description

This PR pins the Node.js version and adds comprehensive JSDoc @example documentation across multiple modules (TokenModule, RecurringModule, EscrowModule) to improve developer experience and IDE auto-completions.

Key Changes:

• Node.js pinning:
  • Added .nvmrc with version 20.
  • Updated CONTRIBUTING.md to instruct running nvm use prior to npm ci.
  • Added Node.js version badge to README.md.
• TokenModule:
  • Added JSDoc @example code snippets to all read methods (balance, balanceOfBatch, allowance, name, symbol, decimals, totalSupply, totalHolders, getHolders, isFrozen).
• RecurringModule:
  • Added JSDoc @example blocks and lifecycle implementations for all methods (getRecurring, setup, execute, cancel, pauseRecurring, resumeRecurring, amendRecurring, transferPayer, getRecurringByPayer, getRecurringByPayee, isExecutable, executeAllDue, cancelBatch, getExecutionSchedule, getRecurringHistory).
• EscrowModule:
  • Added JSDoc @example blocks for read methods (getEscrow, getEscrowStats, getEscrowsByDepositor, getEscrowsByBeneficiary, escrowBetween, getEscrowedValueForDepositor, getEscrowsBatch, isSettled, isExpired, getEscrowAge).
• Changelog:
  • Updated CHANGELOG.md under [Unreleased] with descriptions for all changes.

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Closes #509
Closes #512
Closes #513
Closes #515

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