Add piano key customization and improved styling by CSS variables

[xJkit]

Great library! Thank you for sharing it.

I have made further enhancements by adding more customization options:

Added renderKey render prop api for piano key customization

This API allows developers to decide which keys are disabled. It exposes the entire Key component for greater customization.

Utilized CSS variables

Using CSS variables for styling provides greater convenience for developers and ensures compatibility with modern browsers.

Additionally, I replaced the package manager yarn with pnpm. pnpm optimizes disk usage for node modules more efficiently. The latest versions of yarn (v3+) introduce breaking changes that can be confusing for developers.

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I hope this helps!

Please review the commits. Thank you for this fantastic library.