Issue #783 - Fixed: Missing production dependencies for generating toolbar icons

This MR contains the following changes required for running npm run icons-gen (see #162 (closed)) for building the extension:

• Moved dependencies from "devDependencies" to "dependencies":
  • canvas: Used for rendering the toolbar icons at build time.
  • pngquant: Used for optimizing the toolbar icons at build time.
• Replaced pngquant with pngquant-bin because its binary doesn't exist when calling npm install --no-optional.