feat: Add support for dark mode node icons and colors

[elsmr]

Summary

• Add optional dark mode icons for nodes
• Add node icon color presets for core nodes

// How to define a dark mode icon (backwards compatible)
icon: { light: 'file:googleSheets.svg', dark: 'file:googleSheetsDark.svg' }
icon: 'file:googleSheets.svg'

// How to define icon color from preset (has priority over `defaults.color`)
iconColor: 'blue'

Related tickets and issues

https://linear.app/n8n/issue/NODE-1337/dark-mode-icons-on-dark-mode

Review / Merge checklist

• PR title and summary are descriptive. Remember, the title automatically goes into the changelog. Use (no-changelog) otherwise. (conventions)
• Docs updated or follow-up ticket created.
• Tests included.

  A bug is not considered fixed, unless a test is added to prevent it from happening again.
  A feature is not complete without tests.