Client library file reorg., testing configuration & unit tests (cacheIt)

[matzec42]

• Created a main directory within /src, and moved client-side library (cacheiql-client) files into it for clarity. Updated imports/exports accordingly.
• Set up configuration files, installation of jest, types, other dependencies (like jest-environment-jsdom) for client-side library testing. Tests folder for client-side library is located in the /src folder to avoid Typescript & bundling errors.
• Wrote unit test for index.ts (cacheIt function, which serves as entry point for the library); tests pass successfully (function is executed with test arguments, and function returns a mock result of the cacheiqIt function).