Introduces a feature to dynamically select and display cryptocurrency data (Bitcoin and Ethereum) on the page.

• Added a toggle switch for selecting Bitcoin and Ethereum.
• Added event listeners for the cryptocurrency toggle switch.
• Updates the label text based on the selected cryptocurrency.
• Fetches and displays current price data immediately upon changing cryptocurrency type.
• Modified data fetching logic to dynamically handle the selected cryptocurrency type.
• Ensured the /now endpoint is called whenever the cryptocurrency type is change
• Renamed bitcoin.js to crypto.js

!!! TODO: Inside background.js we need to update the deployed API (maybe we can use a config file to store the base URL)