improvement(v2): customizable code snippet copy button

[kripod]

🚀 Feature

Currently, the background color of the code snippet copy button is not customizable.

Have you read the Contributing Guidelines on issues?

Yes.

Motivation

Some prism-react-renderer themes use a darker background, on which the copy button is barely visible.

[kripod]

It looks like that #2526 has fixed this problem. However, it'd be great to make other aspects customizable (e.g. uppercased letters for "COPY").

[lex111]

(e.g. uppercased letters for "COPY").

For these purposes, you need to swizzle the CodeBlock component, these are very special cases to make them customizable.