Add copy button for code blocks

[NichtsHsu]

Description

Implement #396

Porting from kitian616/jekyll-TeXt-theme#218

Type of change

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• Documentation update

How has this been tested

• I have run bash ./tools/test.sh --build (at the root of the project) locally and passed
• I have tested this feature in the browser

Test Configuration

• Browser type & version: Chrome 92.0.4515.159
• Operating system: Windows 10 21H1 & Ubuntu 20.04 LTS
• Bundler version: 2.2.16 (Windows) & 2.1.4 (Ubuntu)
• Ruby version: 2.7.2p137 (Windows) & 2.7.0p0 (Ubuntu)
• Jekyll version: 4.2.0

Test Result

Checklist

• My code follows the Google style guidelines
• I have performed a self-review of my own code
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• My changes generate no new warnings
• I have added tests that prove my fix is effective or that my feature works
• Any dependent changes have been merged and published in downstream modules