feat: add folder icon for internal folder

[aryaag]

Motivation and Context

• The folder name internal is a common pattern across multiple languages and frameworks
• Specifically, Golang uses the internal folder for internal packages [link]
• But there were no icons associated with it (nor any open pull requests about it)

Changes in this PR

• Add folder icon(s) for internal folder
• Add icons for both open and closed state
• Icon used is downloaded from materialdesignicons
• The color scheme is plain grey as it does not specific to any language or framework.
  • Color scheme is same as the folder-admin icon(s)

Screenshots (if appropriate):

• closed: https://imgur.com/UYwo8lv
• open: https://imgur.com/DCefXoO

[aryaag]

@PKief - any chance of getting this merged?

[github-actions]

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[okineadev]

Suggested change

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