Add JSON Canvas Loader Plugin

[hesprs]

Added a Vite Plugin to load JSON Canvas (.canvas) files. This plugin is packaged together with the main package (json-canvas-viewer), which makes the plugin kind of not fitting the plugin convention since it's not a package. If you mind this, I can make it a monorepo so that the plugin gets a standalone package.

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Edit:

Repo restructured into a monorepo and the plugin gets a standalone package. All requirements qualified.

Checklist

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Plugins/Tools

• The plugin/tool is working with Vite 2.x and onward.
• The project is Open Source.
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• The plugin uses Vite-specific hooks and can't be implemented as a Compatible Rollup Plugin.
• The repo should be at least 30 days old.
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