Add dirty file extensions checker

[fulldecent]

This is a script that runs on the built site.

It simple looks at all the file names.

The script has a hard coded list of unacceptable file extensionns. And matches are reported as an error and displayes advice for that file extension.

Use case insensitive lookup of file extensions.

Example output

❌  articles/horses.png Google recommends to move from PNG to WebP https://web.dev/articles/choose-the-right-image-format
✅ Site includes file extensions: html svg webp

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Following are dirty file extensions:

• .png Only use WebP for bitmap images https://web.dev/articles/choose-the-right-image-format
• .jpg Only use WebP for bitmap images https://web.dev/articles/choose-the-right-image-format
• .jpeg Only use WebP for bitmap images WebP https://web.dev/articles/choose-the-right-image-format
• .gif Only use WebP for bitmap images https://web.dev/articles/choose-the-right-image-format
• .ico (CHECK BROWSER SUPPORT FOR favicon.webp)
• .woff Only use WOFF2 for fonts https://web.dev/learn/performance/optimize-web-fonts
• .eot Only use WOFF2 for fonts https://web.dev/learn/performance/optimize-web-fonts
• .ttf Only use WOFF2 for fonts https://web.dev/learn/performance/optimize-web-fonts
• .htm Prefer to use .html file extension
• .ogv Only use .webm for video files
• .mp4 Only use .webm for video files
• .mp3 Only use .webm for video files