fix: Wait for colors to be loaded before generating preview image

[reobin]

Type of PR

• Feature
• Bug Fix
• Refactor
• Optimization
• Documentation update
• Tests

Description

During preview image generation, wait for colors to be loaded on to the template before taking a screenshot with puppeteer.

Related issue

Closes #415

QA Instructions

1. set GATSBY_ENABLE_GENERATE_PREVIEW_IMAGES to true in .env
2. run npm run build
3. open the ./public/previews directory
4. check out the generated images

Added tests?

• unit tests
• E2E tests
• no, because they aren't needed
• no, because I need help

Added documentation?

• JSDoc
• docs.vimcolorschemes.com (./docs)
• README
• no

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