feat: add CopyButton

[metonym]

Closes #343, closes #351

For highlighted code snippets, a copy button is basic affordance.

From a design standpoint, this library is modularized. For example, line numbers are not included by default to minimize the amount of code shipped to the end user. Similarly, even though a copy button is common, it should be opt-in.

This PR is an initial pass when adding a copy button.

Requirements

• Must be opt-in
• Must work and be composable with all scenarios: Highlight, HighlightSvelte, LineNumbers, with language tag etc..
• Must have reasonable defaults (e.g., a 2 second auto-timeout) but be easy to customize
• Must be accessible/semantic

Implementation Notes

• The difficulty is composition. Because of the modular design of this library, we must avoid using context to auto-detect features.