feat: Theme switcher

[xingwanying]

• Only support g2 & g2plot
• Allow users to switch chart themes.
• Allow users to copy the code of the swatch.
• Allow users to download swatches in png format.

[github-actions[bot]]

🎊 PR Preview d6adafdba51a2f185e600e1f77d340f873b0608f has been successfully built and deployed to https://antvis-gatsby-theme-antv-preview-pr-242.surge.sh

:clock1: Build time: 352.464s

_{🤖 By surge-preview}

[gatsby-cloud[bot]]

Gatsby Cloud Build Report

gatsby-theme-antv

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Build Details

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:clock1: Build time: 7m

Performance

Lighthouse report

Metric	Score
Performance	:large_orange_diamond: 18
Accessibility	:large_orange_diamond: 75
Best Practices	:green_heart: 93
SEO	:large_orange_diamond: 75

:link: View full report

[afc163]

/rebase