feat(tui): highlight hex codes with the color

[jettptacek]

What does this PR do?

Highlights hex color codes in markdown content with their actual color as the background. The foreground text automatically switches between black and white based on luminance for readability.
Supports:

• 3-char (#rgb)
• 4-char (#rgba)
• 6-char (#rrggbb)
• 8-char with alpha (#rrggbbaa)
  Colors with alpha are blended against the theme background, and update correctly when switching themes.

Closes #2586

How did you verify your code works?

Manual verification in Wezterm and Powershell on Windows

opencode_colorcode.mp4

[github-actions]

Thanks for your contribution!

This PR doesn't have a linked issue. All PRs must reference an existing issue.

Please:

1. Open an issue describing the bug/feature (if one doesn't exist)
2. Add Fixes #<number> or Closes #<number> to this PR description

See CONTRIBUTING.md for details.

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The following comment was made by an LLM, it may be inaccurate:

No duplicate PRs found

[jettptacek]

This feature probably benefits from being configurable. I don't know if that is what people want tho. I could see this being part of a set of non-theme colorization options that users could set what they want