Add a shared NuGet icon for the MarkupString packages

[HarryCordewener]

Embed a shared transparent 128×128 NuGet icon in MarkupString, Ansi, Html, Mxp, and Pueblo. The icon uses turquoise angle brackets around three colored text strokes; the existing documentation logo is preserved.

Shared targets select the five published PackageIds and include the icon at the package root. The larger source artwork is retained alongside the package icon.

Validation: packed all five packages from current main (v2.18.1), inspected their nuspec icon entries, and verified that each archive contains the exact expected 128×128 PNG. git diff --check passes. No runtime code changed.

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Review configuration:

⚙️ Run configuration

• Configuration used: Organization UI
• Review profile: ASSERTIVE
• Plan: Essentials
• Run ID: 7253077c-87e1-417c-bdc1-1660d9c4124e

📥 Commits

Reviewing files that changed from the base of the PR and between 0dc73cc and f55a9f6.

⛔ Files ignored due to path filters: 2

• assets/branding/markupstring-icon.png is excluded by !**/*.png
• assets/branding/markupstring-logo.png is excluded by !**/*.png

📒 Files selected for processing: 2

• Directory.Build.props
• Directory.Build.targets

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