Styleable focus outline

[siliconsniffer]

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I used an LLM Chatbot/Assistant during development, but I take full responsibility for the submitted changes.

• Goal of the PR
  Allow the focus outline to by styled to increase visibility or fit to the surrounding style.
• How does the PR work?
  This PR adds a new focus state and properties for color, width and offset.
• Does it resolve any reported issue?
  Closes [Formspec] Ability to style focus outline #16819
• Does this relate to a goal in the roadmap?
  Not directly, maybe 2.3.
• If not a bug fix, why is this PR needed? What usecases does it solve?
  Increased visibility and increased possibility for artistic expression.
• If you have used an LLM/AI to help with code or assets, you must disclose this.
  Disclosed.

To do

This PR is ready for review.

How to test

open luanti → devtest → /test_formspec → press tab/shift+tab → observe that the focus outline changes the appearance

[Wuzzy2]

Please update lua_api.md.

I also suggest to include the * selector in the test since that will likely be the most commonly-used one.