Add TypeScript example of the same accounting pattern to the project

[grahambrooks]

Related to #5

Add TypeScript version of the accounting pattern with tests.

• Customer.ts: Implement Customer class with methods getName, getEntry, post, and balance.

• Entry.ts: Implement Entry class with properties entryDate, entryType, and amount.

• EntryType.ts: Implement EntryType class with static properties BASE_USAGE and SERVICE.

• EventType.ts: Implement EventType class with static properties USAGE and SERVICE.

• MonetaryAmount.ts: Implement MonetaryAmount class with properties currency and amount.

• MultiplyByRatePostingRule.ts: Implement MultiplyByRatePostingRule class extending PostingRule.

• PostingRule.ts: Implement PostingRule class with abstract method calculateAmount.

• Quantity.ts: Implement Quantity class with property amount.

• ServiceAgreement.ts: Implement ServiceAgreement class with methods addPostingRule, getPostingRule, and post.

• TemporalCollection.ts: Implement TemporalCollection class with methods put and get.

• AcceptanceTest.ts: Implement acceptance tests for TypeScript version.

• AnnualServiceFeeRule.ts: Implement AnnualServiceFeeRule class extending PostingRule.

• CustomerTest.ts: Implement unit tests for Customer class.

• EntryTest.ts: Implement unit tests for Entry class.

• MonetaryAmountTest.ts: Implement unit tests for MonetaryAmount class.

• TemporalCollectionTest.ts: Implement unit tests for TemporalCollection class.

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