Fix v1 environment issue

[billytrend-cohere]

This PR introduces a new CustomClient class that extends the CohereClient class. The CustomClient class includes a constructor that performs a check on the _options.environment value, removing any trailing "/v1" for backward compatibility.

Code Changes:

• Added a new file, CustomClient.ts, which contains the CustomClient class definition.
• Updated the import statement in index.ts to import CustomClient from CustomClient.ts and export it as CohereClient.
• Added a new test file, client.test.ts, with corresponding snapshot file client.test.ts.snap, to test the backward compatibility of the CohereClient class.
• Updated the import statement in tests.test.ts to import CohereClient from index.ts.